It's tricky. Prior to my pregnancy, I tried anal sex once after my husband insisting so much. I hate it, it was painful, I truly did not like it. Being perpetually aroused and NEEDING cock (sorry for being so crass but it's how I feel and it's afterall r/sluttyconfesssions) in every hole, it made it easier. My body wants dick and cum in me, so I guess that made my ass more welcoming. That and plenty of lube (though he licked my hole for a while first, then we used lube)

So that, make sure she's very aroused, comfortable and wanting it. Plenty of lube, your dick nice and hard, and voila. Unless you mean tips on convincing her? My husband only convinced me during my pregnancy and I'm not sure you want to do it this way. :D

And thanks! :)