To permanently set the color in the yarn, leave it covered in plastic for 24 hours. Rinse the yarn in tepid water until no more dye comes out of the yarn. Replace the water as necessary. Squeeze out the extra water. Hang it up to dry.
USE THE JACQUARD TIE DYE KIT FOR T-SHIRTS:
Put some ZIP in your day with this easy-to-use tie dye kit. Fun for all ages, this kit is a fantastic introduction to tie dye! You supply the shirts (up to 5) and everything else you need is here, including three pre-mixed colors of Procion MX dye—the undisputed king of tie dye. For use on cotton and other natural fabrics or clothing. For ages 8+. Dyes up to 5 T-shirts. For older kids, this is a fun thing to do at a party. There's loads more information at this link to Jacquard. Here's a YOUTUBE VIDEO showing how to use your Jacquard Tie Dye kit on T-shirts.