Start with a large piece of fabric that is 1.5-2x as wide and your hip measurement and the length you'd like the back of the skirt to be + 1 inch. (My skirt piece was 60"x30" and I'm 5'8".) Fold it in half with right sides together and sew along the edges to make one big tube-shaped piece.
Wrap 3" wide elastic around your waist to measure the length you need, add 1" and cut. Sew ends together as pictured:
(Not Pictured) Sew a straight stitch on the longest stitch length along the top of the skirt, then pull the top thread gently to gather the top of the skirt piece to about half its size.
Pin elastic to the skirt in four (or more) equal places. The elastic seam and skirt seam should match up, with right sides facing.
Sew the elastic on along the top of the skirt piece, stretching it a little as you go to match the length of the skirt piece.
Lay the skirt flat and on its side with the seam on the left side.
Find out how much you want to trim off the top (I trimmed 8") keeping in mind your hem allowance. Cut a 2-3" slit in the front(right side) of the skirt. (This keeps the hemline from coming to a peak in the front.)
Use a measuring tape to cut a diagonal line from the slit to 2-3" from the back seam.
Fold skirt edge under twice and sew to create a tiny hem. This is a little tricky as your skirt is cut in a slight curve now, but don't worry, just take your time and do your best!