wait, is this after a break up, or is this with someone you have never been intamite with before? If it is post-relationship, i dont see how it could be that hard as long as she doesnt hate you and is sexually attracted to you. being FWB is one of the easiest things out there imo. Now if she got too attached to you, she might just need her space. (out of sight, out of mind...)
now if it is with a chick uve never done anything with, then she first has to be attracted to you. I dont see why a chick would want to be in a FWB situation unless its with someone they are physically attracted to. If you two are friends, it shouldnt be that hard to find out if she finds you attractive. Then joke around about sexual things, make her feel comfortable. Ask her if she watches porn and suggest you two watch one together (rare situation tho). just build it up. since you two are friends its ok for you to slightly cross the line (like smack her ass and laugh, or something else like that) if you are playful and humorus
ive been FWB with both types of girls