ideally you will have to freehand it as making a mask will take forever unless you
make a mask that covers a small area and re - use that mask again and again , eventually covering the aircraft.
I did this on the Tamiya HE219 night fighter , the kit supplied with a full camo mask which was copied , all the dark bits were then removed(on night shift)
and I laid the template on the wing .
Not sticking it to the wing gave it a feathered edge.
The other option is trial and error using your airbrish freehand