If the skirt is being shortened, wear the skirt inside out and measure the desired length.

Use sewing chalk to mark the length.

Pin up the fabric along the desired chalk line.

If you are only adding a hem, measure and fold 1/2 an inch up from the bottom of the skirt and pin along the edge.

You want to wear the skirt inside out so the hem is not on the outside of the skirt.

It may help to have a partner mark the desired length of the skirt while you are wearing it.