Mix a tablespoon of enzyme cleaner and 2 cups of cold water in a bowl that's big enough to fit the shirt.
Place the shirt in the bowl, putting the stained portion of the shirt in the bowl first to ensure it soaks in the mixture. Let the shirt soak for 20 minutes.
Remove the shirt from the bowl and rinse the stained portion of the shirt in cold water. Treat the shirt again if the stain hasn't come out yet. Otherwise, launder the shirt in the same manner as you normally would. Check to see if the stain is gone before drying the shirt. Do not dry the shirt until the stain is completely removed -- drying the shirt may cause the stain to become set in the fabric.