It is very simple, actually; when you go to Dudu/Monica, you select "Unit Skill Addition" and you should be able to select a 12* Unit to attach an L-Ring to (make sure the Ring you want to use is unlocked).
The Ring does not take an affix slot, and is just permanently part of the Unit, just like the Potential on a Weapon.
The level of the Ring affects the effectiveness of the Ring's Skill, just like when you keep it a Ring, so you do, in most cases, want to make sure the Ring is at lv.20 before adding it to the Unit (you just don't get to keep the additional bonus stats of the Ring, but those are usually not much to begin with); this means that if you have a Ring whose full effect is unlocked from lv.1 (like Step Jump), there is no need to grind it before putting it on the Unit.
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