(1) Intro to Nudge Arrows/EHT selection menu

The very most important tool for me is the ability to "nudge" décor using EHT arrow keys instead of using the base editor. This enables me to use my "eyeballs" to see what the décor is doing, walking around to view it from all angles while doing so, without my movement and camera controls being tied to actually editing the décor (as is the case with base editor). This menu looks like this:
Basically, you set a keybind in the Controls menu that allows you to select decors into that big box shown in the picture. One the décor is listed in that box, that décor, and any other décor listed in the box, can be modified by the "change these items" drop down menu, or by simply clicking on the arrows shown at the bottom of that menu. You can even change the slider at the VERY bottom of the menu to adjust how far the arrows will move or rotate the decors that are selected. the slider can be set anywhere from literally a 1 integer movement, to a very large movement.

(fun story, you can actually continue to edit decors via the nudge arrows so long as the decors have already been selected outside of edit mode to reduce visual clutter)

(2) Intro to Relative vs Absolute Positioning

This brings me to some very important settings (at least important regarding how I build and approach building, though others use different settings). Firstly, I completely hate that the base game and default EHT setting is "relative" placement. What this means is that, the way your decors move when nudged with the arrows is based on how your character is actually facing (which is how it also works in base editor - everything is done based on how your character is standing or facing). This is a personal preference, I know a lot of people like it, but it drives me absolutely nutty. As such, the most important setting to me is called "directional editing" - I always change it from the default of "relative" to "absolute":
In contrast to relative directional editing, absolute directional editing will ALWAYS nudge and rotate your selections along the same axis, regardless of how your character is standing or facing. This axis is visible via the light blue grid that shows up when EHT is enabled in your game while editing.

(3) Intro to Grid and How to Adjust It

Speaking of Grid, you can adjust it! Most homes in ESO are actually not lined up with the actual "grid" of the map, namely the cardinal directions North, South, East, and West. This is why in many homes, the grid lines go off at weird angles rather than matching the house - its because the grid lines follow NSEW, but the house does not. This is fixable! just click the setting shown in the spoiler, and manually change how the grid lies until the lines match the alignment of your house, and then, very important remember the angle degree of the adjustment to the grid that you made. Write it down if you have to.
Adjusting the grid to match your house's alignment will not only aid in your ability to eyeball whether things are properly lined up, it should also ensure that when you use the nudge arrows with absolute positioning enabled, the selection will nudge along your adjusted grid lines rather than the default NSEW. However this second function is currently bugged. (Cardi is aware of the bug and said he will fix it when he is able).

(4) Intro to Selection and Item Detail Keybinds

Next, if you are new to EHT, you will probably need to set up some essential keybinds. I mentioned in #1 that you can use a keybind to "select" decors into the main EHT selection window, and once selected, decors can be altered using the arrows and change these items features. If you have not already set up this keybind (named "select/deselect"), you can do so in the setting shown in the spoiler. Additionally, I have highlighted two other keybinds that I consider essential to how I build: "show selection window" and "edit item details":
"show selection window" merely shows the regular EHT selection window, or if it is already open, closes that window. I keybind it for convenience because, a lot of times I don't actually edit via a selection, as described in #1 above, but rather by the edit item details menu and, unlike selecting an item, using the edit item details button will not by default launch the selection menu as well.

"edit item details" is a very fun editing tool for anyone who is obsessive about perfect placement and alignment. I will describe it in #5, below. For purposes of setting this keybind, using the key you bind to this control while hovering over a décor in edit mode will select that décor into the edit item details menu. (very similar to how the "select" keybind will select into the selection menu shown in #1.

(5) Intro to Edit Item Details Menu

The edit item details menu is probably the most responsible for how I am able to almost perfectly align decors. To use it, enable editing mode, hover your reticule over the décor you want to edit, and use the "edit item details" keybind. This will open the edit item details menu with the décor you targeted selected.
As you can see from the picture, there are a number of extremely useful features for creating perfectly positioned items in this menu. You can directly input specific cardinal values to change the items position. You can even copy those values with the control + c function, select a different item, and paste that value in. You can also enter rotational values, or, if the selection is an FX, you can alter the size or color of the FX.

Remember how I said in #3 that you should remember the degree of the adjusted gird you ended up setting? Here is where that becomes important. In my picture, Mistveil manor is actually on a 36 degree offset from NSEW. As such, when I want to place an item, I simply select the item into edit item details and enter 36 degrees as its rotational value (typically. though not always zeroing out the pitich and roll values, at least to start with). This ensures that the item is already lined up to a 90 degree offset of how the house is aligned.

At that point I then typically move to the nudge arrows in the selection menu. This is where we get into one of the peculiarities of the edit item details menu: whatever item you have selected into edit item details will ALWAYS supersede anything you have selected into the regular EHT selection menu. This means that you can use the nudge arrows to edit the item you selected into edit item details without selecting it into the selection menu. In the case of the column I selected in the screenshot above, since the fireplace I was making was 180 degrees offset from the alignment of the house, I likely simply selected the column, set the angle to 36 degrees, then changed the slider for the nudge arrows all the way to max (on max, a single click of a rotational nudge will rotate the selection exactly 90 degrees). nudged it twice, and that is how the item ended up at a 216 degree offset. (BTW I am really just generalizing. In truth the column would have been part of a multi-item selection copy and paste of a previously created half of the fireplace... but this is more advanced that this introduction to editing tools is designed for).

Using the edit item details menu in this way ensures that all decors are placed perfectly straight, perfectly rotationally aligned, all with the same "z" value (or height). After that, it becomes much easier to use the nudge arrows to carefully align the items - especially as on the smallest setting, the nudge arrows will only move the décor 1 integer.

VERY IMPORTANT when you are done editing the item details, CLOSE THE EDIT ITEM DETAILS WINDOW. As I said before, the edit item details selection will ALWAYS supersede your EHT selection. As such, if you do not close it, and then try to go back to editing groups of items in the selection window, instead of moving the selected group, you will continue to only move the edit item details selection. See spoiler for example.
Also note, unlike the EHT selection menu, edit item details can only select one item at a time. It cannot be used to simultaneous edit a group of items, unless that group is all linked and the parent décor is the one that has been selected into edit item details (note if doing this, I always recommend un-linking the items after you are done, as linked groups can sometimes bug and move themselves in weird ways from time to time without being edited).

(6) Intro to Saved Selections, Copying, and Pasting

Lastly for my basic intro to features I use most often, I just want to briefly mention creating saved selections and the copy and paste functions.

If you have spent some time, or even just a little time, cobbling an item, creating and copying saved selections will be your very best friend. (it's also often how I actually create complex cobbles in the first place, but I'm not getting into that here, since this is just an intro to features).

For example, the living room chandelier in this home is made of 40 individual decors. It would be a huge pain in the butt to have to select all 40 items every time I want to move that chandelier. Further, what if I want to make a second chandelier that is an exact copy? I certainly would not want to spend the time recreating it from scratch.

The solution is saved selections:



In the picture above, you can see that I have selected all 40 items that make up the chandelier (well, you can't actually see it, since you would have to use the scroll wheel to scroll down, but I assure you that I have indeed selected them all). After selecting them all, I then typed a name into the "selection name" box; in this case, "living room chandy" is the name I chose. I then hit "save", which is just to the left of "delete" and right above the selection name box. Saving the selection does two main things: (1) the selection becomes selectable again by hitting the drop down menu under "revert to last save", meaning even if I have cleared my selection or whatever, I can always just, select "living room chandy" from the drop down menu and immediately start editing this exact group of items with the nudge arrows or "change these items" function; (2) it gives you access to the "revert to last save" failsafe, which, not matter how the decors have been moved or edited, if you revert, the items will go exactly back to where they were at the last time you saved that set.

For example:
Selections and saved selections are also very useful when it comes to the copy and paste functions. There are number of ways to access copy and paste. For example, you can literally click "copy/paste" which is above the "pitch" arrow in the EHT selection menu to launch the full copy and paste menu (but this menu is far more complicated than is typically needed). You can also hover over the same shortcuts icon I identified in #3, but instead of clicking the adjust grid option, instead click "copy items", and then "paste items", to copy and paste your current selection. Or you can set a separate keybind to copy and paste in the controls menu (this is the method I typically use). However you do it, copying will make a copy of all decors selected and their directional and rotational positions. Pasting will grab a copy of all decors (though they have to be in your bank, storage chests, or inventory) in that selection and place each of those copies in the exact place saved in the original selection that you copied, and EHT will automatically create/open a new selection containing only the copied (not the original) decors. You can then use the nudge arrows and "change these items" functions to edit that newly-copied group and move it wherever you want it to go.

Note, you can even copy and paste between houses by using some of the features in the more complicated "copy/paste" menu, or paste from house items and other areas instead of the default inventory/bank/storage chests as well. However, the keybind copy and paste option, and the shortcuts icon copy and paste option, will always paste from inventory/bank/storage.