Slow & easy. Bring it up in passing, not pushy. Guage her reaction.
Plant the seed & let it grow in her imagination.
Mention it again, days/a week later. Keep it gently on her mind. If she talks about it, still, go slow. Tell her u just think its sexy.
Take her out for drinks. Mention other guys at the bar. Slowly get it to be a common, non committed topic.
Until she shows interest. Perhaps as ur fucking? That always a great time to tell her how hot she'd look/be, fucking another guy.
But most of all, absolutely convince her that its OK with you. That u have no problem with it, in fact u think its hot.
Not tried & true for every woman. You know how she is & will react. Just go slow & easy, baby steps to start. Good luck!