I've seen it done a couple of different ways:
One is to make a feathered edge at the border between two colours with a dry brush technique using one or the other colour.
Another way I've seen is to put the camo down hard edge and VERY gently stroke the edges of your camo areas with very fine steel wool or sandpaper. You'll really want to let your paint cure well before trying that. If you use acrylics, I'd say substitute the steel wool or sandpaper with an old toothbrush.
It would help to know exactly what you're puting the camo on: Tank, aircraft... and what era its from. Different eras and different nations had different camo patterns. How you accomplish the pattern depends largely on what the pattern is itself.
As for the "wet edge" method, don't try it on models, its really is meant for traditional art mediums like watercolour paper and canvas that are semi absorbent. I doubt very much you'll get it to work on non absorbent surfaces like styrene.