Lining a knit scarf will make your scarf warmer and hide less-than-perfect stitching. A liner will also force a rolled-up purl-stitch scarf to lie flat, increasing its surface area and usefulness. Polar fleece is often used to line knitted scarves because of its ability to stretch with the knitted fabric. Polar fleece is a thick material and works well in cold weather. A polar fleece lining will increase a scarf's ability to keep you warm.
Lay your scarf onto a flat surface. If the edges roll up, weigh them down. Measure the length and width of your scarf. Add an eighth of an inch to all measurements to account for a hem. Write these measurements down.
Using a pen on the underside of the fleece, mark out your measurements. Cut the fleece as indicated with scissors.
Place the piece of fleece on the back side of the scarf. Turn the edges of the polar fleece under by an eighth of an inch all the way around. Pin the polar fleece to the scarf with straight pins. This will result in a knitted border around the edge of the polar fleece.
Thread a sewing needle. Knot and cut the thread. Sew around the edges of the polar fleece along the straight pins. If you are about to run out of thread, knot the thread attached to the needle, cut it and re-thread the needle with new thread.