Painting

Painting your stencil is the final step.  You've done all the work to create your stencil and now it's time to put it somewhere for display.  Depending on your project, there are a couple of different application methods and an unlimited amount of surfaces for you to paint.  Generally, your surface needs to be generally smooth or your stencil is not going to turn out how you want it.

If you're painting your walls in your house with decorative stencils or quotes, you'll want to use a small paint roller (like a 3 inch trim roller) and a bucket of paint in a tray.  Using spray paint in your house or apartment is not something I would advise.

If you're working with canvas, cardboard, concrete, metal, wood, etc; you'll want to take it outside or in a well ventilated garage (or art studio) and use spray paint.  If you're outside, pay attention to the wind and get your can closer if there's a breeze.

Cover your mouth and nose and wear rubber gloves.  You might get some paint on your arms.

Mount your stencils to the surface being painted with a light mist of spray adhesive. Don't spray too much glue on it.  If you're doing multiple layers, I would advise not using adhesive after the first layer.

Use flat spray paint, don't use glossy spray paint.  Don’t let wet paint dry up on the stencil, you can lay it on some newspaper and wipe it off with a paper towel.  Be careful not to tear it.  One thing you can do is put one hand down on the stencil and peel it off with the other hand, being careful not to let the stencil slide and smudge your design. The best paint to buy is Montana Gold, but any rattle can of Krylon or Rustoleum will work as well.

If you’re doing multiple layers let each color dry before you start on the next one, otherwise you're going to smudge the wet layer. Depending on conditions, this could take 20 minutes or it could take an hour or longer.  It's important not to get impatient or you will wreck your picture and possibly have to re-do the entire design.  Find something to do while you wait for it to dry.  Each layer will take a bit longer because the paint underneath it will become wet as the layer on top of it is applied.

In general, I think it's better to underspray your designs.  Too much overspray or getting too thick with your spray paint doesn't look very good in my opinion.  You might have a different opinion and you're entitled to it.

Copyright

I'm no lawyer, nor am I a copyright lawyer. That said, I am in no way qualified to give legal advice, but this topic is something that should at least be touched on.

If you are planning to sell a stencil or a painting made from a stencil you've made, make sure you have the rights to use the picture you made your stencil from.  Even selling a drawing based on a photograph which you do not have rights to use can get you into hot water if it's not in the public domain.  Even if a photograph is in the public domain, if it has an identifiable person in it you need to get permission.  There are publicity and privacy rights that are separate from copyright.

There are a few websites out there with good information regarding copyright and public domain works.  Public domain sherpa is a good one I suggest you look at.