Spray the back of the wood sign 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 t-shirt to protect the back 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 sign sticky side down on the t-shirt and press well so it adheres.
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 sign. I started by spraying inside the sign, then sprayed around the outside of the sign, trying to move around the sign 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 sign.
As you wait, you will begin to see the spray lightening up the fabric around the sign. Be patient and don't spray more immediately as it takes a few minutes to see where you have sprayed. After 5 minutes or so you can add more spray in any areas it seems like you have missed. Again, do not soak the fabric.
Allow the bleach spray to dry for 20-30 minutes, then gently peel the wood sign off the shirt.
Immediately rinse the shirt in running water until the entire shirt is very wet, including all the parts that were bleached. Rinse and wring out the shirt a couple of times, then wash and dry it on a normal cycle in your machine before wearing.