I don't remember exactly either, but it is the ending I got first by following my instincts, here's the rules I followed:
Don't do anything that involves other people having sex with her, maybe play on the idea of other people looking at her, but don't let anyone else touch.
Stick to your guns and remain in control of the situation while training her, if you deny her when she wants you, don't let her convince you otherwise.
Don't cum inside her until you can get her to beg for it, then once you do cum inside her, convince her to have sex with her husband to make sure the timing lines up and he thinks it's his.
Follow those general rules, and I think you should get there. I agree it's my favorite and the best ending.
