What is the best way to explain on open relationship?

I have flirted with this one girl in my History class for the first 3 weeks of the semster. I deffintly have her attention and esablished a good repor(tho im not sure how sexualy charged it is) and she is coming over to hang out of my place later on this week to watch a few movies(this is the first time we plan to hang out outside of class, and I do have some highly sexualy charge commieds) But she already knows I have a girl friend, however she does not know me & my girlfriend have an open relationship. so my questions are
A.
What is the best why to bring up the subject?
B.
After I tell her, what are things I should look for to futher getting her into bed?(This is a very conservative area so she mite be repulsed by the idea)
C.
I im not looking for a romatic relationship with this girl, just sexual friends, any tips on how to do this?

BTW id gage my chances of eventualy getting her into bed 70-30 in my favor, though mybe not this week. I just dont want her to get to attached.

im not exactly an expert on this situation...but my guess would be to sleep with her first...if you played ur cards right than she probably doesnt care in the first place....once uve slept with her...as the alpha in the situation you explain to her HOW the relationship is gonna work...and if shes really attracted to you..she wont care....just my 2cents

an update ?

hi JW85,

any updates on this situation ? I know it's been a long time, I just wasn't on the forum until recently again...how did it go ?