For starters, do not completely blindside her by proposing anal sex completely out the blue. You have to warm her up to the idea slowly over time. So, if you haven’t already, start making jokes about trying anal here and there. You know, just lighthearted stuff to sort of plant the seed in her mind (in hopes of eventually planting the seed in her behind). Chances are, if you get her accustomed to the notion ahead of time, she’ll have had time to consider it a little bit, even if subconsciously. Then, when the time comes to actually propose it, it might feel just a little more comfortable, familiar to her.
You should also never just try to ram your dick in her asshole during sex. This is a big no-no and will result in you probably never being granted access through the elusive backdoor at all. All this will do is shock her and cause her a great deal of pain. Then you’ll just have a pissed off girlfriend on your hands. And nobody wants that. In fact, this will probably even put a stop to the sex you were having, then you’re left with blue balls and a week of the cold shoulder.
Once you’ve warmed her up to the idea a little bit, though, over the course of several weeks, you can start incorporating the butthole a little more into your regular sex routine. Tease her asshole with your finger, with the head of your dick while you fuck. Eventually, one day, she might just find that she likes how that feels. Let her guide your dick into her ass at her own leisure. Getting your girl to try anal is an art form and, thus, it cannot be rushed. Take your time and, if you do it right, eventually it will pay off.