Turns out this is actually pretty easy. If you want to see a bit more tutorial, please click here.
Freezer paper can be found in the plastic wrap/sandwich bag section of your local grocery store or Walmart. One side is paper, the other side is waxed. You can find stencils or silhouettes anywhere on the Internet or clip art sites. Transfer your design and then cut out the space that you want painted. Fabric paints specifically for screen printing work the best--look for these at your local craft store. They tend to be a bit thinner and don't clump on the surface.
100% cotton works best for your surface. Always pre-wash to get the sizing out of the fabric.
Iron your cut-out on what you want to stencil, glossy wax side down. It will now stick to your fabric and create a moisture barrier so the paint spreads no further than desired. Let it dry for a few hours (2-4) and then peel off the paper. Give it a few more hours and then heat-set the paint with an iron. Use a cloth over the stencil when ironing. You can wash your item after a couple of days. I'd probably turn the item inside out and be careful when drying.
Cool custom designs await you, but a freezer stencil is only one time use.