The legend goes, "Anal sex is cool, every guy must try it". Well, it is really a kind of taboo, a very cheeky thing to do and this excites. But before you start convincing your girlfriend to let you in her tight asshole, you should know how a woman feels during anal sex. Many girls find it tight and not pleasant. They say that it feels like a muscle cramp. Some of them feel pain during the process. So before you ask her girlfriend, think first whether you want to get her in trouble. But if you are sure you want it, you can try to convince your partner.
Show your best diplomacy skills, because it often happens that women don't want to tell how they feel about it, or just shy, or have health problems that can make a big obstacle. You need to know the real reasons. When you do, it will be easier to convince her.
The most common reason to say "no" to anal sex is the woman's fear. They fear pain. They are afraid you won't find her clean enough. They fear you will think bad of her, knowing she is curious about assfucking.
What can you do about it? First of all, tell your girlfriend that you love her as she is, even if she is not clean. Second, assure her that you will stop if she feels pain. And, of course, give her some time to think it all over and prepare herself.
Meanwhile, you can take some steps yourself. Note what exactly in your foreplay makes her really hot. You can use it to feel her more relaxed on that special day, when you try anal. Also, start to prepare her during your usual sex. Reach the zone between her buttocks with your hand and insert a finger. Do it slowly and gently. And yes, do not forget to do your nails before!
So what is her reaction? Does she likes your finger inside her bum? If yes, you can try the same with a special sex toy next time. It will prepare her to feel a real penis inside. Be gentle. Do not forget that your life is not a porn movie where everything is easy and possible.
What else? You need a lot of lubricate. Anus do not produce own juices, so have to make it wet on your own. Then choose the best position. Usually, it is the one that gives control to your woman. And enjoy it!