Spray the back of the wood heart with basting spray or other washable adhesive spray until it is slightly sticky to the touch. If you don't have basting spray, you can use a glue stick instead. See note.
Place a piece of cardboard inside the sweatshirt to protect the back (or front) of the shirt from the bleach spray. Ideally, the cardboard should be big enough to fill the entire shirt. Place the shirt down on a trash bag or something else that will protect the surface in a WELL VENTILATED area (outside is best as long as it isn't windy). Place the sticky side of the wood heart down on the shirt, pressing down to make sure it is well adhered.
Make sure the spray nozzle on your spray bottle is turned so you get a fine mist (test this with water). Mix 1 part water and 1 part bleach in the bottle and begin spraying it over the heart. I started by spraying inside the heart, then sprayed around the outside of the heart, trying to move around the heart in an even circle. You only need to get the fabric lightly wet, so DO NOT soak it with the bleach water. If you do you will ruin your shirt because the bleach will begin to leak under the heart.
Bring your sweatshirt inside and let it dry - you can use a hairdryer to dry it more quickly. Once it is dry you will be able to see the fabric around the heart has gotten lighter. Gently pull the heart off the sweatshirt to reveal the design.
Rinse the sweatshirt in cool water for a few minutes. Then wash in a washing machine to remove all the bleach and adhesive. Dry in the dryer (or hang dry) then it's ready to wear!