I am really getting some bang for my buck when it comes to the stencils I bought a few months ago. My students have used them to add pattern to large portrait paintings and spray paint ornaments. In clearing out closets at school I found some boxes of unused t-shirts and aprons that we decided could be used for a fundraiser.
Spray worked on the tight weave of the apron, but didn't show up well on the knit t-shirts, so we used acrylic paint on both. We taped the stencils down with packing tape to give a wide enough border for mistakes to be made. A stencil brush is ideal, but a foam brush is the second best thing. The trick is to dab up and down rather than using strokes which cause the paint to be pushed under the stencil plastic and mess up the image. Things really got fun once students started varying the colors within a single stencil, such as adding more red to the orange or a lighter green to the outsides of the stenciled image. It ended up being a project that can be done within an hour, teach medium and contrast, and still have a wow factor.