To Stencil a shirt with fabric paint, you have to Tuck a piece of cardboard inside your shirt. You need to Place a stencil on top and secure it with tape if needed. You must Apply fabric paint using a sponge brush. You should work your way from the edges of the stencil inward. You can Let the paint dry, then peel the stencil away.
For a trendier look, you have to use fabric spray paint instead.
For something more unique, you can use bleach spray.
You need to Create your own stencils using freezer paper.
