I have stencilled quite a few seats. I do have a vinyl cutter and sign software though which makes things much easier.
For stencils,I have found that the better 7yr vinyls are softer and stick to the textures that some covers have better than others.
As far as Paint goes, I use thinned down Screenprinting ink, the type used for stickers and the soft Vinyl wheel covers
you see on 4wd's etc. I spray with an airbrush or small touch-up gun. A couple of light coats to reduce the chance of the ink
bleeding under the stencil.
CG, message me if you need a hand.