My bf wants a finger in his butt

My boyfriend has talked about how past partners have done this and he’s liked it, and I think he wants this done to him. I want to, but I want to feel confident in technique before I do. Is in best to just put one in there? Do I put the whole thing in there or just tease? How much movement feels good? Do I need lubrication to do this ? Are there any good positions that would feel really good to do this in? (While giving him head or while I’m on top etc.) any advice would be appreciated I have a hard time trying new things without feeling like I’m going to do it well.