When you want to shrink a cotton shirt, the key thing is heat. You can shrink white or colour shirts with or without lettering on them.
Using the hottest water setting, wash the shirt by itself. For colour shirts add one cup of white vinegar to the wash. The vinegar will lock in the colour and keep it from bleeding.
After the wash cycle, remove the shirt from the washer. If the shirt has any lettering or images, turn it inside out.
Using the highest heat setting on your dryer, dry the shirt. This will yield a shrunken shirt.
If you want to further shrink your shirt, repeat these steps. Be sure to let the shirt soak in hot water during the wash cycle. You could also soak it in a pot of hot water on the stove.
-Before attempting to shrink your shirt, make sure it is 100% cotton. Non-cotton or blended shirts will not shrink properly and could become damaged.