The method to use depends on the type of camouflage that you want to do - soft or hard edged. There are a number of different ways to do both, here are a couple (I'm assuming that you are using an airbrush?);
Hard edge: The easiest way to do this is with masking tape, using either thin, flexible vinyl masking tape (often used for automotive detail & pin-striping work) or Tamiya type tape. Depending how tight the pattern is that you want to mask, you may have to slice the tape a bit thinner than standard, so that you don't get any lifting or wrinkling at the apex of any corners.
Use the fine stuff to mark the outline of the pattern & then fill the remaining area in with broader tape or paper.
Soft edge: Blu-tack / White tack / Poster tack cab be used here. Just roll the tack up into long sausages & apply these over the camo demarcation line & fill the remaining area in again with tape, paper, whatever. As you have made the tack round in profile when rolling it, you can get varying degrees of sharpness depending on what angle you airbrush from, if you paint from straight above the tack you'll get a soft edge, if you paint from the masked side toward the tack it's an even softer edge & if you paint toward the masked side you will get a sharper edge. Playing about with the diameter of the "sausages" will give you even more flexibility regards how soft or hard the edges will come out.