Asking for a detailed description on how to steal a girl from a guy sounds like a bad joke. It hardly deserves an answer, but I will give you one anyway. However, it won't as detailed as you want since people are people, and not robots.
First, it depends on the girl, the guy, the relationship they have, the situation you meet them in, and finally on you. Only from this it becomes obvious that a detailed answer is impossible.
Second, if you have read the book The Art of Seduction you will know what the prerequisites for seducing her are. If she is in a happy relationship with the guy you can just forget it. You will not succeed under any circumstances.
Third, assuming they are not happy together, or has some other (non sexual) relationship, you do as one who is a master PUA would do - just be a social and charming guy. Not detailed enough I suppose, so let's delve deeper.
First you open, saying whatever is appropriate. Most guys will fuck up here since they have missed to OFFER some value. Why would they want to talk to you? So you need to do it without wanting anything, not even the girl. You just want to have fun. Then you focus on both of them and make sure the dude likes you - become his friend! Become the girl's "friend" too. The only difference between "being just friends" and being a sex partner lies in the vibe, how manly you are. So check out RSD Julien's videos on this. Once you are friends with both, having a fun night with them, all you need to do is to be more fun and stimulating than the dude. Just remember your rapport - you need to calibrate you humor and energy to the set, be at their level and just a bit more.
That's it from.me. You have to do your homework on building rapport, being physical, flirting, speaking with the right tonality, giving her the right vibe and everything else on your own. Otherwise I have to go into ALL details and theory about game and that is a huge subject. Checkout
www.pualingo.com for starters.
CHAT