I put my POP in a tupperware, twice the volume I think I'll need, and then add water slowly and mix until it's all wet and the consistency of pancake batter. Let it sit for a minute, so any little bits of unmixed material gets wet, and you should be good to go.
Cold water gives you more working time.
Vibrate gently to eliminate any air bubbles that get trapped in the pour.
I typically coat my masters with POP by hand first before I lay them into the mold, so I'm sure all the details are filled.
Take your time, you should have lots of working time, and make the mold face as even as is possible, if you're making a two part mold.
Someone here (I apologize, I don't remember who) posted a link to his website, where he has a tutorial on POP mold making. Do a search and see how he does it.