The put on command is, by default, mapped to shift + P
If you choose to put on a ring, you may be asked:
"Which ring-finger, Right or Left? [rl]"
Press r or l to choose, as indicated.
You are unable to put anything else on when wearing an amulet, two rings, and one piece of eyewear (e.g. a blindfold). Additionally, you can't put rings on if you are wearing cursed gloves or wielding a cursed two-handed weapon. If you are wielding a cursed one-handed weapon, you can still put on or remove the left-hand ring; all player characters are right-handed.
You do not have to Take off your gloves to put a ring on. Strangely, you also do not need to take off your gloves if you Wore them after putting on a ring, either...
- Remove, the inverse command