There is a advice for beginners,
lube , circles on the ass, lube, finger sliding from front to rear over the ass (scrotum to coccyx) with a increasing pressure. And suddenly the finger enter, be soft, be slow, be lubed, this cuddle must be nice not a rape, after one finger, use your thumb, still slow soft and well lubed, the receiver must move the muscles of the ass like needing to go,after the thumb, the light is green for two fingers .
Go slow, take time, but do it twice or three times a week just like an exercise, no target to reach, just to be both confident and more relaxed in the soul and with the ass, and share
you the ass of your partner ? give yours first ! never ask what you have not "taste" !
Enjoy