Zok:
I did mine with a soft edge and I did not use any masking. However, if you wanted to do the hard edge my only experience is when I did my Challenger. I used ordinary masking tape for masking. I have read around here about using "silly putty" but I can't find that here in my area.
When I did my Challenger, I did not mask the whole thing right away. I worked on specific areas until I completed the whole thing. I cut the 1" masking tape with irregular edges and lightly stuck them on the area I am working on.
I just made sure that the paint is not too wet and I shoot it with the airbrush nozzle direct perpendicular to the tank surface so that no paint will go under the tape. I also sprayed lightly each time making sure that the paint is thin and will not flow or make the masking tape edges soggy or wet. Then since I am using acrylic, I can immediately remove the tape as the paint usually dries fast.
Also one thing that I have learned around here is to work my way from the lightest to the darkest colors.
Good luck.
BTW it should be NATO Green and NATO Brown, sorry.