OK, first, have a look at post :
#11,489
Edit your save and get the Hands off event. It raises Rachel's submission by 1 each time you surprise her.
And/or : Continue to have sex with Rachel at night with Amy watching, until Rachel reaches submission level 4.
Then go talk to Susan, you'll have a dialog chain to start the 'Master' event, that allows you to spank Rachel. Catch her disrespecting you and enjoy it.
When her sub is high enough, you can call Susan to watch. It will get you Susan 1rst level.
Then give Susan the rare seashell, she will trade it for a spanking