Welcome Rommel. As far as what to do, well it's going to come down to you really. Trial and error will decide what works best for you but if you decide on a particular course, by all means we'll help. I do my camo's freehand but other's like to use a soft mask or hard mask so it's hard to say so it this way. One suggestion though that's pretty rule of thumb. When painting camo's, spray your lightest color first and work to the darkest. So Dunkelgelb, then Red then Green on the Marder. That shows the least overspray that way.
Petbat and Lizardqing gave 2 good examples of a soft mask. A hard mask would be taping a mask or something similar.
The best thing is to try one way this time, and another on your next model to see what works for you. And once you decide what to try, let us know, we'll be glad to offer more opinions!
Eric