When I was painting I would just lay the stencil down and spray. I usually had a couple and some I did cut.

I think someone else mentioned it but defintly scuff the body using a scotch brite pad, avoid the windows . Then wash and let dry, avoid touching the body and spray the liquid mask.

For backing, I used to use spray can bed liner on 1/8 bodies. Worked pretty well, but it can adhear to the liquid mask on the window and can be a real PITA to get off. I switched to Rustolium Clear Enamel. Works well.

I would suggest buying some sheets of lexan from Amazon and test different backing methods, this includes E6000 and Shoegoo as they can react with paint and eat a way it.