Note: this is only for ranged weapons!
My attempt on a tutorial video:
https://youtu.be/hwd3RwbvepA

Written Guide:
Basic Function:
Normally the function of this skill is that after it's activation your next Light attack thereafter that hits an enemy procs additional damage. However for bows, restoration staffs and destructions staff there is a way to buff the light attack before the skill rather than the light attack after that skill.

Melee Light attacks vs Ranged Light attacks:
Melee Light attacks are instant light attacks. Meaning that if you do a Light attack using 2h, Dualwield or Sword & Board it will connect near instantly when you press it. On the contrary the ranged Light attacks from bows and staves (all staves) have a travel time which is independent of it's distance. So it doesn't matter if your standing directly beside your target or at maximum distance it always comes with the same travel time.

Why does that matter?:
The game has blessed you with a technique called "animation canceling"! Let's say you do a Light attack using a flame staff then you activate elemental weapon. Now the travel of the Light attack ends before the full animation of Imbue weapon so the Light attack does not get the buff. But "Animation canceling" be praised you don't have to endure the full duration of elemental weapon you can cancel it to shorten it's animation. Meaning you can buff the previous Light attack if the animation of Elemental weapon ends before the travel time of that Light attack ends. In fact there are four ways to achieve this: dodge roll, weapon swap, bash, and block.

Animation cancel techniques:
-dodgeroll cancel: this technique shortens the animation of an ability by immediately doing a dodgeroll after the animation of a skill. so if you intent to do a dodge-roll then use it besides that use another technique.
-barswap cancel: If you intend to barswap after the use of elemental weapon this technique is for you. It is done by immediately barswapping after the use of an ability. If you don't however intend to swap bars then use one of the last two techniques.
-block cancel: Done by shortly blocking right after the usage of a skill. However be aware that blocking cuts your stamina regen so you might miss on regen ticks if you block while a regen tick is happening. And of course you might be charged stamina if you block while an attack is hitting you.
-bash cancel: This is the ideal technique when there is no target in bash range. Since bashing the air doesn't have a stamina cost attached to it and does not impair your stamina reg like blocking. Be aware that as soon as there is a target in bash range this might become costly. however if you are using a bow you might use it to squeeze in some extra damage (usually done on Sword and board). Bash canceling is done by immediately bashing after using an ability.

Where is this relevant?:
In PvP.
You can avoid delay when using Imbue weapon: This makes this skill more bursty and helps you to follow up with further damage after you applied your dot's. You can still choose to do the classic technique to have like a "bursty" Dot application by going Imbue Weapon -> buffed Light attack -> Dot, but after that you can immediately follow up with additional imbue waepons.

It is more efficient use of global cooldowns:
Imagine following: you want to hit an enemy using elemental weapon. but he dodges your Light attack. In case you have empowered the subsequent Light attack the buff is lost. However if you have empowered the previous Light attack you have the buff still active even if the Light attack misses. So you don't need to activate the ability again you can instead cast another ability and still have another Light attack in store buffed by Elemental weapon.

Don't screw up weaving especially in pve!
Have your animation canceling under control. Especially in PvE either animation cancel every elemental weapon or cancel none. If you don't hold onto this you will waste elemental weapons from time to time resulting in a dps loss.

Hope this was helpful. Since it seemed to me that this interaction between ranged weapons and Imbue weapon was mostly unknown to players. This interaction makes me consider Imbue to be one of the best if not the best spammable option for most ranged builds.