How do I tell a girl I want to sleep with her?

One good way is to ask her what she thinks about sex. That gets the topic out in the open. If she seems comfortable with the subject, you could ask something like what she thought about having sex with you (not at that time, but in general). That should give you a firm grasp of where she stands with being intimate with you.

Dont press the issue, and if she gives some response you don't like then you might just have to settle for the long haul.

Sex is sort-of the next step after making out. If you've been making out a few times, she's probably comfortable enough to venture into that territory, and it wouldn't be out of character to ask her what she thinks about having sex with you.

If it hasn't already been discussed, and you're not in the kind of situation where you might get intimate... asking her if she wants to have sex in the 'present' tense isn't a good idea.

It really depends on the nature of your relationship and how close you are already.

If you're boyfriend and girlfriend but haven't been that intimate - only light kissing or holding hands, or just a few dates, then sex is a big step. You should probably try to do other things like making out, cuddling, staying over at her house for the night (but not sleeping together) to get closer to each other.

You have to remember, having sex is a very intimate and sensitive thing for a girl. It's not something she's going to treat lightly.

Then again if she likes sex, or has been secretly been wanting to sleep with you for awhile... she might be like "Finally, what took you so long!". And in that case it's not a bad thing either because it shows that you have some integrity and didn't just want to have sex in the first place.