The most important thing, if all you're doing is leveling, is to know how experience is distributed. Whenever you gain experience, it will be allocated to the individual skills you have equipped, the skill lines those skills are in, the weapon(s) on your active bar, and the skill lines for each piece of armor you have equipped, proportional to the number of armor pieces of a type equipped.

Some skill lines only gain experience from doing specific things (like reading lore books for the mages guild skill line), and companion experience works a little differently, but for leveling yourself up, just keep the armor/weapons/abilities equipped that you want to level up.

As an example, if I still want to be mostly effective as a magicka templar, but still level up my two-handed skill, I'll just replace an ability I don't use much with a two-handed ability. Even though I won't be able to use the ability, every time I get experience I'll be getting some of it put toward two-handed, without ever having to switch my whole build around. Another common example would be making sure to equip at least one piece of light armor, one piece of medium, and one piece of heavy, and eventually all will get to level 50.

Other than that, just try to make sure your armor and weapons having the training trait on them (crafted armor is great for training) - I recommend finding a grandmaster crafter, and they'll have the knowledge to help you best with your individual situation - and use any experience scrolls/psijic ambrosia/aetherial ambrosia/mythic aetherial ambrosia you have.