Just have her perform tasks that you set out for her. This is where you need to get creative. We can only suggest things, you know this girl better than us so you should be thinking about the kinds of things she might like. If she likes to be humiliated then work with that.
One girl I used to know said that one of her favorite experiences from her master was when he made her smear toothpaste al over her vagina and ass. Not my thing, but it worked for her. Get creative, get messy, get commanding. Don't make her suck your cock until she meets your criteria as a dirty cock sucker.