Nothing like a good rimjob.
“Preparing” is pretty simple.
Clean your asshole like it’s you that’s gonna put their tongue there.
If you look around you will see the ones that make sure you are clean going in, in the shower pre session.
If you look even closer you might find someone that will walk you through the internal cleansing process (enema).
I have done this with a now retire pro Domme before a rather intense first pegging session, no one wants to see or smell anything gross during a session so if you’re gonna get pegged or fingered consider an enema before a session, you can buy enema kits at your local drug store or on line.
If you’re gonna just get fingered then a good self fingering in the shower pre-session is a good idea. Some research on your part will help you with diet pre session (yes, a considerate participant will regulate their diet prior to bum play).
Rimming is just fingering butt more fun.
The key for you to satisfy your sphincter is to find a provider who does rimming and will join you in the shower where you have to communicate what you want and then give permission or encouragement to get your butthole to a place where your partner wants to lick it.
Enjoy!