I need advice

Hey so my boyfriend has always been really nervous for me to finger his ass and never really showed much of an interest in it but the other day he mentioned he’d like me to do it. Does anybody have any tips on how to do it well or just any advice on what I should do? I want the first time he experiences it to be good for him and not one he regrets, and also one I won’t be too bad at because I’ve never had anyone that’s wanted it xx