Wearing a crotch rope, is one of the best things invented for a woman when It comes to bondage.... besides a few other naughty things
I for one suggest for your first time, you do it with her panties on... so she can get the general feel for it, Its better to ease someone into something new than just to throw them in, in the deep end!
Secondly, I suggest you use a soft rope, bondage rope or cotton is that best! If you use rope, thats scratchy and horrid, then the pain would be excruciating than pleasurable!
Thirdly, the tightness, most crotch ropes are tied to other bonds, like a rope around the wrist or to the hands, this in effect can give the captive some control of how tight the crotch rope is pulled. You should always make sure that it is not only secure but safe, you don't want to be hurting her now... I mean she wont thank you in the long run!
I suggest you build it up, work on it, start with the basics and work your way up the ladder to the top.. If she feels uncomfortable at first, thats a good sign in a way, because she is not used to it, and so you may need to loosen the rope, or use less strands etc.
Once she is used to it, then you can progress more and more, by adding knots to the crotch rope adding more strands, the tightness!
But people will have limits, and the limits you must obey by... If someone does not like something, you stop immediately, its a golden rule, should never force someone to do something that they do not wish to do! If she keeps complaining, saying its painful, then the crotch rope is not right for her! #