Because acrylic yarn is made of plastic, knitters and crocheters often mistakenly think it can't be blocked. Blocking is the final step required to make your handmade objects take the proper shape. While acrylic will not stretch as readily when wet as cotton or wool, you can coax the fibres into shape with steam. If your acrylic beanie is only slightly too tight or too short for your head, steam blocking may stretch it just enough to make it fit.
Cover your bed or couch with a folded bath towel.
Stretch the beanie as much as possible with your hands, then lay it on the bath towel.
Insert metal T-pins all around the edge of the beanie, sticking them through the towel and into the bed or couch. Place one pin every inch, stretching the beanie as you go.
Fill the steam iron with water and allow it to heat. When the iron begins to produce steam, pick it up and hold it 3 inches above the beanie. Do not allow the iron to touch the yarn as the yarn will melt.
Press the steam button and hover the iron over the beanie. Steam the beanie for three minutes, keeping the iron moving the entire time.
Leave the beanie to dry overnight. Remove the pins the following day. The beanie should hold the shape into which you pinned it.