I think the best first step would be to broach the subject of bdsm with her in a safe and comfortable environment, and measure her reaction to it. If she is not open to it, immediately drop it. It's no good to try and force a lifestyle on somebody. Wait until she brings it up before broaching the subject again. If she does seem to be open to that sort of experimentation, be very polite and ask if she would be willing to try it sometime with you. Always make sure that she is comfortable and not feeling 'weirded out' or nervous. Try switching with her a couple of times, because she may not be a sub, and don't always be intimate with her only in that way. Be sure to keep vanilla sex on the table so she doesn't feel forced to have 'kinky' sex with you. Always make sure she knows she could stop at any time, and, if all goes well, after a few months of experimenting with bdsm, ask her if she is willing to try to take it a step further. Again, if she says no, back off. If she says yes, start communicating with her about how you would like her as a sub. Being easy, slow, and gentle is key throughout the entire process. I hope this helps.