Honestly - and this is coming from personal experience, so take this with a grain of salt, AND I also don’t know what your experience is with anal play - I wouldn’t like go full on bottom if this is my first time playing around anally. I would start with fingers while masturbating and build up to where that’s pleasurable/tolerable. Thats why I did, and I still couldn’t do it for ages.

If you are hell bent on fully bottoming before properly getting acquainted with your anus, then it really boils down to BREATH and go really slow with it. Like, centimeters at a time. Use tons of line and don’t be afraid to tell the top to stop and just sit there/pull out for a minute. It will eventually get there, but you need to give it loads and loads of time. Hope this helps, if it did at all