To ensure the threads have the opportunity to shrink fully, make sure they are clean and wet. Use hot water to make your garment pliable for the drier. Wash the garment using a full wash cycle for maximum effectiveness, even if you're only washing the item you want to shrink.
As soon as your garment is finished in the washing machine, dry it in the dryer on highest heat level until it is completely dry. Don't forget to use a fabric softening dryer sheet to ensure the garment is soft, wearable and free of static.
Once the garment is fully dry, check to see if it has shrunk to the desired size. If not, repeat the process until the garment is acceptable. If your garment isn't shrinking to your liking, boil water and soak the garment for 10 to 15 minutes. Carefully transfer the garment to your washing machine using gloves or tongs, and repeat the hot wash and hot dry cycle.