The degree to which clothes shrink in the washer and dryer depends largely on the type of fabric involved. Cotton, wool and linen shrink quickly when heat is applied, while synthetic fabrics are much less likely to shrink.
Wash the garment in hot water
Make sure the garment is machine-washable, then start the shrinking process by putting it in at the hottest setting available on the washing machine.
If the item is already the size desired after washing, line-dry it, or use an air-only or fluff setting in the dryer. Otherwise, set dryer heat at high.
Check the garment at regular intervals during the drying process. As soon as your item has shrunk sufficiently, turn dryer temperature down, or take the item out to dry in open air.