I'm agreeing with Clancy here. The game you are about or think you are about to start has being ongoing since you met her. Just because you've been siting on the bench doesn't mean that the game wasn't going on.
Think of it like this. You are just coming into the game. A rookie off the bench. She on the other hand is a vet and has been in the game for a while. It's going to be a bit more challenging for you to do this without a doubt. BUT its possible.
First of all, like I said before, you are already making a huge rookie mistake. You are thinking too much about it. STOP THAT. I am going to quote cheech and chong here and I may be the first to do this on a PUA site

"CHILLLLLLLLLLLLLLLL OUT MAN"
You have the skills/ knowledge to do this, everyone does. So when you get into the next session, take your lesson. Don't think about her or f'ing her or how hot she is or annything like that. If you aren't paying attention shes not going to want to be there. So really listen to her.
A little while into the lesson relate something she says to her. Make it random and off the cuff. If you do it right she will begin to talk about herself in some way. Thats an in, jump on it, don't think. Use this as your time contraint. Suggest that the two of you take a short break, but that you can't do it for long cause what she is teaching you is important. This makes her feel like you respect her, so it lowers her guard and it sets the time.
After this you can run some game on her. I don't know the girl and I don't know you, so I can't tell you what will work exactly. But using what I just gave you to set it up should work perfectly.