Can you explain your question more?
Is this for a heterosexual couple or ?
What is the ages of the couple?
What exactly are you wanting to know?
Anal sex can be enjoyable by both partners if you are very careful and considerate of her needs.
The first step is a lot of foreplay.......anal sex should not just start. You go forth as you would regular intercourse with lots of attention and foreplay. If she is not 100% in the mood this is going to be uncomfortable and even possibly painful for her. Once she is 100% in the mood Anal penetration can be started.
The biggest key is using lubrication.....do not use Vaseline or oil. There are lubes such as KY Jelly for this matter that work the best. You will want to put some on her as well as some on you. In the doggie position is best......pentration should be very slow and easy. If she has pains just stop for a few moments and allow her to adjust to you. When you start again go slow......the first few times will have to be very slow for her to not be in any discomfort.
If she would like, you can pick up anal toys at places such as the lions den or online to Begin with experimentation. This will allow her to loosen up some and not be as worried about it.
The biggest factor is lubrication and ease.....the slower you go the better this will be for her.