First, cut your fleece into long strips. The strips are about 4-5 inches wide and 5 feet long. You'll want to cut it shorter for a child size scarf, about 3-4 feet long.
Layer 2 or 3 fleece strips on top of each other. It's more fun if these are each a different color of fleece. Then sew down the middle, just one straight line. This is best done with a sewing machine, but you could simply use a needle and thread if you're without a machine.
Cut 1 inch wide slits up each side of the sewn fleece strips. You want to get pretty close to the middle, but DO NOT CUT through the sewn line.
Now fluff it up and it's ready to wear!