Start slow real slow, try inserting a lubed finger as you get used to that, you can get a small probe, or plug, again go S-L-O-W. and use lube, make it enjoyable and work your way up to larger things. Did I say SLOWLY.
My wife has recently become quite the anal-queen. This comes after 23+ years of saying no - but that's another story.
I do two things to get her ready. 1st I make sure she has 1-2 good blended orgasms combining vaginal and clitoral stimulation. When we're ready to move on to anal, I apply plenty of lube - then slowly insert a finger allowing her to adjust She's experienced enough now that it doesn't take long - but as a beginner the pause is important. Once she is comfortable with my finger, I reapply more lube and work in the Tantus silicone B-Bomb. It's a modest 1.25" at the crest - then necks down for comfort.
Pausing again to allow her to adjust to the larger size - I massage her vulva. Then I will slowly work the B-Bomb out to the crest - then back in. This allows her anus to get use to the larger size.
When she says she's ready - we prop her hips up on the liberator wedge pillow - because she prefers missionary (as opposed to doggie or cowgirl). This allows her access to her clitoris either manually or with her Mystic Wand.
Just before penetration I apply a liberal dosage of silicone lube. It eases entry and never looses its slippery feel.
She's become adept at having an orgasm with our 'routine'.
Enjoy - it can be quite rewarding if you and your partner exercise a lot of patience (go slow).