My girlfriend had no sexual experience when we first met... but was open to trying anal. With her both due to her size (she's 5') and inexperience I worked her slowly up to actual anal sex over about a month.
Started with a finger massaging the anus during sex a few times to start, then had a very non-intimidating anal exploration toys that came in three progressing sizes and had a very minimal circumference. These looped onto my finger that allowed us to use both during sex and on their own. I would always start back with the smallest toy and only work up to the next larger size we hadn't used after ample use of the previous size. All the while playing with her clit, massaging her breasts, or fingering her vagina if we weren't having sex while using the toy.
This small starter toy set of plugs was a good starting point, we also continued building up to use a larger set of trainer plugs (Small plug: 4.5” x 1.1”, Medium plug: 5” x 1.25”, Large plug: 6” x 1.5”). We've never used the large plug from this second set.
I also got a flexi felix
set of anal beads for further anal play/warm up. I'd also sometimes warm her up with a butt massage that then got heavier towards massaging her anus leading up to using any toys/fingers for penetration too.
Using these toys, which allowed us to slowly progress in size both in width/insertion slowly over a month period lead to anal sex just being another progression and not a sudden intrusion. But like many others have said, lots of lube is important too. We use copious lube with the toys and my fingers as we warmed her up for penetration, and I also use a lube launcher to inject copious lube internally before I try to penetrate in addition to more on my penis too.
Hope that helps, but take it slow, use adequate lube, and make sure your partner feels safe and able to give feedback if going to fast or it's too much to fast.