Half circle skirts are the best sort of a-line. The seams are on the grain, so they sew up without pulling and make it relatively easy to put in zippers. A half circle is a nice amount of fabric for an every day flared skirt. It’s not a dramatic as a full circle, which makes it easier to wear. And a half circle pattern is very easy to draft.
To make a pattern for a half circle skirt, you will need your waist measurement and the skirt length. In this pattern the front and back are identical. For patterns this simple I generally just draw on the fabric with a piece of chalk. For each piece, you will start at a corner and draw two quarter circles – a small one near the corner for your waist, and a larger one farther away for the hem.
Divide your waist measurement by 3 to get your waist radius (people who like trigonometry may be tempted to divide by pi, but rounding down helps balance out the stretch along the bias angled parts of the circle). Hold one corner of a yardstick or tape measure at the corner of your fabric and measure out your waist radius. Swing the yardstick along the fabric, keeping one end in the corner, and mark out points as you go. For example, if your waist radius is 10″, you would mark out several points 10″ away from the corner of the fabric at different angles. Connect these points with a curve.
Add your skirt length to your waist radius to get your hem radius. Mark out points at this distance, as you did for your waist. Cut along both curves. This is the front of your skirt. Do the same thing at the opposite corner (or trace the piece you have) to make the back.