It takes a lot of time, lube, and practice. I am also very think and long. I have always loved anal sex, it feels great having my cock fully penetrated in my partner. I have been working my big cock up asses since I went to college many years ago. Since then every partner I have had has taken me anally. Typically if my partner has not had a lot of recent anal sex with thick guys it takes a long time.
We recently went on vacation with a couple that we are bringing into our circle of friends that we have group sex with. It was the first time that either my wife's friend or her husband had succeeded in having anal sex. The first time it took me 30 minutes of slowly working my cock in and out of her before I fully penetrated her. He is very large also, after I had screwed her ass he was able to screw her ass also, but it still took him about 10 minutes to fully penetrate her. Later that evening I screwed him, and it took me about 25 min to penetrate him.
I suggest having your partner on her back with the legs spread. This way you will be able to see her reactions, and the two of you will be able to communicate better. Start with lots of good lube and keep on adding more to your cock, I even pull out every so often and add more directly into my partner's anus. I like to use KY Ultra Gel on my cock, and use KY Jelly for direct application to the anus.
Work your cock in a little and then back it out, repeating over and over, each time going a little deeper, very slowly. Have her bear down, as if she is taking a dump, when penetrating deeper meet resistance. Some thick guys find that just the initial penetration can be difficult. I like to start with my fingers and smaller toys before I start to penetrate her with my cock. She may also like to have a vibe to work her clit while you are doing her ass. I have partners that can easily orgasm during anal sex, other women like the vibe to help them to climax.
When I do begin to insert my cock I start with my thumb, or a couple fingers extending past the end of my cock head. My thumb will penetrate first and then I find it easier to have my head follow my thumb in. As my head begins to enter I withdraw my thumb.
Having your partner very relaxed also helps. If she is all up tight and nervous, no matter how much time and lube is used, you may never be successful. Some couples like to use alcohol, others use poppers, I suggest staying away from either of these. Anal sex requires cooperation and communication between partners, using these stimulants reduces feeling, responsiveness, and communication. I suggest a nice hot, relaxing spa tub bath. Set the mood with soft music, low lights, and candles. Make this experimentation very romantic for her; it can tend to become too mechanical. I find this approach makes my partner very relaxed.
Good luck and let us know how the two of you progress.