The key to anal is lots of lube and take it slow. Also it's so much better if she's relaxed, so I recommend to get her off first, specifically by eating her ass. When you're going in, just tease it with the tip at first. Then slowly work your way in. It'll let her prepare herself and it'll be more fun for you to really feel the tightness of her ass. Take it slow and gradually just start going harder and faster. If she's still in pain after have her lay on her side and give her some ibuprofen.
Sorry this is so long I am just giving my piece. I think everyone should enjoy anal