I know i know, this is completely backwards and you shouldn't get into a relationship with someone you just want to sleep with, but how would you do it if you realized you wanted to?

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How to Get an Awesome Girlfriend
How to Train your Girlfriend

Ha! Good frickin luck on that one.

I'm not sure how you would go about this one, depending on how long you've been in a relationship, how long are we talking - days, months or years?

If you've decided to become official, you'll need to break it up with her and explain why, try be honest with her, but don't ever say "its not you, its me" or some other cliché reason to break up with her.
I'm not going to lie to you it probably won't be easy, especially if she's got feelings for you.

Treat it as any other breakup, freeze her out for a week or two (maybe more), work on yourself and then reinitiate contact.

I've used the above with an ex and we're on speaking terms, we've even hung out once or twice since, but I've never decided to pursue her with a 'lets be f buddies' approach. If anything, it probably would mess with her head and her healing process.