Hi there.
I personally have never used the stencilit products, but I have used something very similar. I have the Lion Roar U.S. stars Photo-etch stencils and have used them to good effect...
LionRoar Markings For U.S. Military Vehicles Review on Armorama The Basic use is that you cut the P.E. Stencil off of the fret and tape in place on your model and airbrush the markings on.
Like decals stencils have their good points and bad points...
If you want to put a stencil on an area with complex shapes or details in the area the stencil will not lay flat and you will most likly have underspray under the stencil..
If the area is fairly flat the better the stencil will lay and the less of a chance of underspray..
I have given some thought to the matter of preventing underspray and i've considered using a liquid masking film as a glue to attach the stencil to the model to stop underspray.. but I must admit that I havn't tried this yet so I have no idea how well it would work...
The best results using stencils are to attach the stencil to the model and spray very light coats straight down on the model. For stencils you don't want to angle your airbrush as that can cause overspray and too thick of a coat can cause thick paint lines..
check out my
My M10 GMC page for an example of a model with stencil stars painted on...
hope the info is some help to ya..