I would practice without any wake at all - you can do a perfectly great nose stab with an all stock ski with the right technique. Practice slaloming and unweighting the turns like snow-skiing: zig-zag left and right sinking your body into the apex of the turn and unweighting as you come out of the turn. Just get into the rhythm of it - imagine there is a line drawn in the water stretching out in front of you and your are zigzagging back and forth across the line, but each time you cross it your want your weight off your ski. Down in the turn and oopla! up over the line. Don't try to jump the ski, just get used to that unweighting feeling.
Once you've done that for a bit, just go ahead and on one of the left turns steer hard left and pin the throttle - don't do anything different with your body apart from your regular sinking in the turn and unweighting over the line. It's all about timing - hot the throttle "on the line"- when you get it right you will be shocked at how the back end whips around without even trying. Do that a few times to get your timing down and then you can be a bit more aggressive - throw your head at the gas cap on the left turn where you hit the throttle. Boom! nose stab, without even a setup wake.
I prefer a nose stab to get vertical - rear up, nose down. But, if you're going for that invert feeling then you then need to start on getting your weight forward. For my money a Jason Stoyer style stab is the best - body straight, back arched, ski vertical, not upside down.
Now you should start hitting the wake which will actually set you back a bit since it's much harder to get the timing down. You need to be curving to the left up the wake so that you unweight the back end as the pump intake reaches the lip. Just practice practice practice and COMMIT! You don't want to flat land a stab or you'll be nursing a badly sprained wrist for weeks.
If you hit a cruiser wake then everything is in slow motion: big S turn up the wake - hardly any body motion, just get your head forward somewhat and pin it. I love the floaty feeling of a big air stab. Try to tear your eyes off the front of the ski which will help your body position. Start working on look-back stabs as you get more confident - they look cool, but the also help get your style nailed. The best stab ever is in big surf where you stare down into the trough behind the wave for what seems like an age - awesome feeling!