A couple of ways around that.
A lot of guys here swear by the "blue tac" method. Roll blue tac into thin sausages, mark out the camo lines with that and then fill the gaps with paper, tape, tissue or what is close at hand.
Lay the tape on a clean surface, bit of glass, a tile, cutting mat. Some thing that you can peel the tape off! Cut the curve with a scalpel lift off and put in place.
On Large things, I have found pvc insulating tape pretty good, soft not to sticky and stretchable ie you can bend it. Good for long curves that are not to tight. The sticky can leave a mark but I have not had problems with it.
And so the most risky one. You need a frightfully sharp blade and a gentle, steady hand.... Put the tape over the line you wish to mark on the model and cut through the tape. ONLY the tape. Good side it you get the curve right and in the right place in one go. bad side if you use to much pressure, you WILL cut the paint under the tape. You really need to just scratch the tape 98% through. Not easy but like all things, practice makes perfect.
Oh one last thing. If its quite a large subject why not just free hand it. Its not impossible again its down to practice and a steady hand.
Ian M