Like everyone else has said, going super slow is key. Going from personal experience, I would recommend prepping for a good while before you attempt to penetrate with your penis. The pain or discomfort is caused when the anal sphincter muscles are not relaxed and relaxing them can take quite a while, especially if she is even slightly newrvous and new to anal. Building up slowly and using lots of lube is key. You could spend time with one finger inside, moving up to two fingers slowly and taking the time to let her bum become accustomed to the new sensations each time and gradually relax. Wearing a butt plug would be ideal, especially if she wore it for half hour or more before sex, but if she does not like it, then the prep will have to be done with fingers, slowly, painstakingly slowly, over the first hour or so of your session.
Another tip, from experience....When you start to penetrate her, please remember to use some backstrokes at the beginning. I have done anal with a few exes and my current partner and one common complaint I have is, in their desire to get inside, they just push forward continuously, pushing and pushing. It kinda pulls at the anus in an uncomfortable way! In other words, I don't like it when his first stroke is all about going balls deep without also pulling back a little to allow my bum to get used to the sensation. It requires a little force to push inside yes, and as soon as the head slips inside it can become intense very quickly....if he carries on pushing and pushing at this point...it is too much. Stop after the head is inside, even pull out a little, then on the next stroke inside you can sink a little deeper, so long as you do everything sloooowly. She needs to be relaxed too and over time, her bum will relax more and accomodate you, allowing you to thrust faster (This may not happen in the first few sessions though. I think it took me a good 5 to 10 attempts at anal, before he was able to begin thrusting inside me and even then it was slow thrusting. It was only after much more experience did it feel comfortable for him to thrust hard. It takes time.)
Painstakingly slow....gallons of lube..and not forcing anything inside are all key!