Wash the sweater and air dry it. This will remove any residue that might prevent even dyeing.
Fill a sink or bucket with hot water. Using a bucket will prevent you from having to wash out your sink with bleach to remove the dye. Use a bucket that is large enough to immerse your sweater in.
Dissolve powdered dye into 2 cups of water. Use the appropriate amount of powder for the amount of water you are using. Consult the manufacturer's instructions to determine this.
Put the sweater into the hot water and squeeze it until it has absorbed as much water as possible. This will enable even dying.
Add the dye to the water and stir constantly for 30 minutes.
Remove the sweater and rinse it with warm water. Gradually adjust the water so it is cold and squeeze the water out of the fabric. Continue rinsing it until the water runs clear.
Wash the sweater in warm water according to the care instructions. Let it air dry.