First off, if you genuinely have zero interest you shouldn't feel pressured to do it, like at all. I know you say part of you is interested, but just want to make sure that's not you telling yourself that because you are worried you'll upset your partner.

Now in terms of if there is actual interest, I can give a bit of a advice from my life:

• start with just a finger and lube. If you can do it together IRL it's really the best way. Relax, breath, and try to get used to the sensation. if he knows what he's doing he'll be very gentle, and honestly this simple step can go for a long time so feel no pressure to "train up"

• look into plugs for alone time. You can often buy them ina variety of sizes, and work your comfort level up over time.

• when having anal sex for the first time, don't be scared to say it just isn't working. If you get nervous and freeze up, he won't be able to get his cock inside you more than likely. Genuinely do not sweat this, it happens. Shake it off and try to explore it other ways and revisit.

• finally, just accept it's a bit gross and learn to laugh if it goes wrong. I've genuinely been pooped on before by the sweetest human on earth who didn't know what was happening until it happened. Stuff happens, of you two have a good bond it's not gonna be the end of the world, just another funny story between the two of you.