Butt plugs are objects with a round knob that goes into an asshole and a flared handle that prevents it from getting stuck inside. This latter part is why you should really stick to butt plugs when plugging butts, if you use a dildo with no such flare, or one of many wacky objects people like to insert in asses, the object sliding in and getting stuck is a serious risk.
Its use is simple, lube it up generously, and slide it inside the ass. Ideally take it out and relube every 20 min. There are a variety of sizes, so one can pick what is best, or have a few to work up to.
Some of them have an attachment, for instance a tail, and as such can be used, in this case, for pony play.
Once it is inserted, you can sometimes press on it to “remind” them of the sensation. Then do as you like, roleplay, foreplay, sex. Sex with a butt plug for her feels quite intense and well-thoroughly-fucked, and a pleasant arousing supplementary kinky stimulation for the man.
A fun domming thing to do is to make your bottom wear the butt plug when you leave home for a short period of time (for instance to go to a shop to get some drinks). It’s a naughty but secret sensation that makes the experience nicely charged, and the dom can, when no one is looking, come behind and press on it.
I met a girl once who worked at a sex shop who told me this funny story: once a gay man with a heavy German accent, yes think Bruno, came in asking for dildos, and then asked for the biggest one, and asked if there was anything bigger, there wasn’t so he just took the biggest. He then said that he intended to wear it while he was driving.