
Whether or not you like the idea, anal sex is very popular - almost two thirds of couples have tried it or do it regularly (and that's just the ones who admit it!). And with good reason, too, for the anus is rich in sensitive nerve endings that can add an extra thrill to the pleasure of sex - not to mention the fact that something so forbidden, something so naughty, takes us into a whole realm of taboo-breaking.
And why's it so popular? Well, in any position sex up the anus has one big advantage for both men and women - it's tight up there! There's nothing a man likes more than a tight hole to penetrate, and they don't come much tighter than the anus. Of course it wasn't made for entry, just exit, so there's some skill in reversing the traffic up this one way street: and sex positions are a more important factor in anal pleasure than in any other form of sex.

If you want a straightforward anal thrill, the classic rear entry sex position is the way forward, so to speak. If you're experienced in anal sex, you'll be able to push in slowly, using lots of lube, until you're past the resistance of the inner anal sphincter. There may be a feeling of needing to poo, but hopefully this will pass quickly. If your partner's not into douching, you might want to use a condom to avoid getting shit on your cock. If you're not experienced in anal sex, you might want to try with a finger first, to get the feel of it.

As your partner relaxes, you can gradually insert more than the tip of your finger into her, moving it around in circles as you do so. You can gently begin to stretch the anus, moving your fingers around in a circular pattern as you do so.
What you're aiming for is to get her relaxed enough to be able to take your penis without clenching up. Her main fear is that she's not going to be big enough, or maybe that she'll shit on you. You'll need to reassure her that this isn't likely to happen - and even if it does, so what? You can easily clean up in the shower.

As always, athleticism counts for a lot! If she's flexible, and you're; strong, this can be a great way to enjoy a vigorous session of anal penetration....
If you're not, you'll probably want to try something more achievable like the anal version of the side by side sex position as shown here.
And it follows that you can do anal while you're lying down, too. Since a lot of women feel very vulnerable in this position even when they're being penetrated vaginally, you might want to give some thought to reassuring your partner and telling her how much you love her (and how grateful you are for the privilege of being in this position!)