4949 -
A few things I've learned (and have been noted by others):
(1) take your time taping. A mistake there negates a lot of good work elsewhere.
(2) practice on something less important first. I learn something every time, and I try to apply those things in subsequent projects.
(3) the distance of the reverse stencil (grass in my projects) from the gun or scope determines the clarity of the shadow effect. Spraying over something leaves an outline, but if that something is very far away from the gun the effect is "fogged" quite a bit. Conversely, a reverse stencil that is tight to the gun leaves a sharper "shadow", or highlight. Neither is right, it just depends on the effect you want to get.
(4) the left and right sides of the gun will be easy. Transitioning to the top will be a little tricky. It can be challenging to hit the top and still leave your prior work intact. That part just takes practice I think....
(5) and finally, take it easy and enjoy the process. Or like the book I read on how to brew your own beer used to say, "Relax. Have a home brew." If you don't like what you see, just put a new layer of base on, wait 20 minutes, and re-stencil.
Have fun!