There is a whole different set of protocol for picking up chicks if you yourself are also female.
First thing to remember is, you must feel out whether or not said girl has any inclination towards her same sex. Many different ways to do this. One is by becoming friends and spending time together and basically beating around the bush. The pun isn't intended, but it seems to fit well right where it is. Or you could take the direct approach. Women can do that, there's a lot less hemming and hawing between females. You can point blank ask her if she's ever been with a girl. You can ask her if she were a guy, but I hate this one, what would she find attractive in other women? You could be even more direct and ask "Hey, baby, wanna screw?", but this would merit a slap in the face just as if you were a male predator.
So you've determined she likes the ladies at least on occasion as well. Ask her about her experiences. Ask her when it happened, how, where and with whom. How did she feel about it? Would she do it again? Share some of your experiences. Drop names of actresses or singers who you think are attractive. See if she reciprocates. If she does, do they match you or at least your taste in women?
If she hasn't grabbed you, flung you onto her futon, ripped your little Incubus band shirt off over your head and proceeded to introduce herself to your nipples, then you know you're going to have to take the lead.
Look for the right moment. Look for what her eyes tell you. Is she comfortable casually touching you, sitting next to you? Does she giggle a little more than you would expect at your jokes? Does she go out of her way to graze your arm with hers, does she just need to hug you for no apparent reason? BINGO
The rest is just nature and timing. You can take her by the hand and lead her away from other people. You can ask her if you can kiss her, that seems to work well. If you need more nerve, you can take a sip of something alcoholic, but not too much, because you're gonna want to remember this.
WARNING It is a rare occasion when sex is just sex between two girls. Emotion is a huge player, so tread carefully, you don't want to squash something important.