Laminate (optional but it will last longer if you do multiple shirts)
Cut out laminated part from pattern.
Place old gift bag or cardboard inside shirt
Position your template (tape down if needed)
Using bleach pen, color in the design
Let sit for a minimum of 10 minutes. (you can go longer for a lighter design and outcome will depend on the color shirt you begin with)
Rinse completely.
Air dry.
Notes
I recommend using 100% cotton if you plan to use bleach and remember that the effect will be different depending on the color shirt and the length of time you leave it on. If you want a specific color, fabric paint would also work.