Shrinking clothing is not an exact science. It is difficult to precisely shrink clothing by exactly two sizes. Shrinking clothing can be done at home using a washer and dryer. Heat is what makes clothing shrink. For example, you should easily be able to shrink a shirt from a size large to small in one wash cycle. You may need additional washes in order to shrink the item further.
Place the shirt that will be shrunk in the washer.
Set the washer for the hottest, longest setting. You can add detergent, but it is not necessary. Wash the shirt.
Remove the shirt from the washer after the wash cycle ends.
Insert the shirt into the dryer.
Set the dryer for the hottest and longest setting. Dry the shirt.
Remove the shirt from the dryer. The shirt should have shrunk. If it has not shrunk enough, then repeat the process.