Take a few of the C clamps that you squeeze the handle to tighten and place two of the clamps about where you want the rocker to start, then wedge a 2x4 or similar piece of wood between the tips. Ive found the the more exaggerated and closer to the C clamps you put the 2x4, the better it works. Eventually when completed and the block is removed the rocker will settle way out.
Use a heat gun to head the rocker area, not to hot to start to make the ski really hot, just kinda warming up the resins a tiny bit. Too much heat can cause the ski to start to delam. Flip to the other ski after about 15 mins.
I did this for about 2 .5 to 3 weeks on a pair of pink and green line motherships. It actually worked decently well, added a subtle obsethed-ish rocker. I tried to do the same to the tail, but because it was stiffer than the tip it didn't do much, except maybe flatten out the camber? Eventually after about 30 days of solid use the ski went to pretty much 0 camber, totally flat. Wish I still had them!
Some of the guys on TGR have had better luck keeping the added rocker when the ski in use has some metal in it. Pretty simple and fun experiment if you have an old pair of skis laying around! What skis are you thinking of using?