Place several layers of newspapers inside the shirt to keep the mud from soaking through to the back of the shirt.
Think up an Earth Day slogan you would like to have on your T-shirt. (Examples: I Dig the Earth; Earth Day, Every Day; Dirt Shirt for the Earth.)
Dip a stick or your finger in the mud and write your slogan on the shirt.
Let your artwork dry. Shake off any caked-on mud before proudly wearing your dirt shirt. The design will fade in the wash. But you can always reapply.