After you finish knitting a scarf, make it lie flat by blocking it. Blocking serves two purposes -- it gives your knitting a bath, and it relaxes the fibers of the yarn. If you knitted your scarf in stockinette stitch, in which you knitted all the right-side rows and purled all the wrong-side rows, your scarf is likely to be curling in on the edges and forming a tube. Blocking it will flatten it out.
Soak your scarf in a sink full of cold water with a cap full of wool wash for at least 30 minutes. Rinse the scarf with cold water and press out any excess water on the edges of the basin.
Lay two large bath towels end-to-end on a bed, rug or carpeted floor.
Lay the scarf out on the bath towels and smooth it flat.
Pin all the way around the edge of the scarf, inserting one straight pin every three inches.
Allow at least 24 hours for your scarf to air dry. When it is completely dry, remove the pins. Your scarf will lie flat.