Slow and steady wins the race, especially in anal play. So before cracking out the strap on with huge dildo, play about with smaller, easier to insert things such as fingers, anal beads and plugs designed for anal play.
Make sure you use lots and lots of lube. You can’t use too much. The anus isn’t self-lubricating so you need to keep reapplying lube throughout play. Pick out a dildo that your partner is happy to take. Always start out small, you can get bigger and wider at a later point. Keep communicating. If they say stop, stop. It may take some time before you can fully peg your partner, but the patience is well worth it!