Lay out the sweater, the bottom will be the waist of your skirt.
If you have an old skirt you would like to use as a pattern, line up the top of the skirt with the sweater. . . If you don't have a skirt to use, try the sweater on and pin the sides then carefully take it back off.
After pinning, cut off the excess fabric, make sure the right sides are facing in, and sew with a zig zag stich.
Try on to make sure the skirt fits how you would like then hem the bottom with a zig zag stich and viola! :)