1. Leave the stencil on and put the glitter on top.
2. Take the stencil off and put the glitter on top.
I went with choice number 1 so that I would have nice clean line edges. If you want your edges to be softer then choice number 2 is better.
I hope you will have a chance to play around with some plastic stencils and make some marvelous glittering creations of your own!