You can shrink new articles of clothing---or clothing that is supposed to be line dried or dry cleaned only---with a hair dryer. The advantage of the hair dryer over a clothes dryer is that is more gentle and gradual, so you will not over-shrink the clothes. It could take a while depending on the thickness of the clothes, so make sure your arms are rested and ready to go.
Wash the article of clothing you wish to shrink.
Check the label for the type of fabric and care instructions. If it's supposed to be dry cleaned only, it's an expensive item or it's made of a particularly delicate material such as thin silk, washing it may damage it. If you're concerned about damage, test wash a small hidden portion of the clothing.
Instead of washing, you can also dampen the clothing by dipping it in water or spraying it with a water bottle.
If the article of clothing can be dried on high heat and it isn't new, it will not shrink when heat from the hair dryer is applied.
Hang the wet clothing on a hanger. If it is made of a thick material, let it air-dry partially until it is just damp. If you try to blow dry it while it is completely wet, the first 20 minutes will be ineffective and tiring. If you like, you can throw the article of clothing in a regular dryer on low heat for 10 minutes to get the excess water out.
Turn on the hair dryer and aim it---preferably with a nozzle attachment to concentrate the airflow---at the clothing from about 6 to 10 inches away. Move the dryer up and down slowly. Try to cover the entire item of clothing every minute or so. You can keep it moving constantly, or you can focus on smaller areas for longer if it's made of a thicker material.
Stop and check the fabric every 10 minutes or so to see how much it's shrunk. Be careful not to over-shrink. If you are trying to shrink a particular area more (sleeves or waist, for example), focus on that area for longer.
Don't work next to water such as a sink or bathtub in case you drop the hair dryer.