I'd turn it inside out and put it on, pinch the sides or back until the waist fits comfortably, then use safety pins to hold it together. It would be best to make this fold along a pre-existing seam.

After that take the pinned skirt, place it flat, and use something (washable marker or a sliver of soap work well) to draw a making line from the top of the waist to the fold. Use the safety pin as a guide for where to put the line.

After that you can just sew a straight stitch along your line all the way from fold to waist. Make sure to use some good sturdy thread. This is easier if you have access to a sewing machine, but if you don't and need to hand sew it here's a tutorial I found for a simple straight line.

After that try it on again and if it fits you can cut away the excess fabric. Otherwise, use a small pair of scissors to cut your stitches and redraw your line to be further or closer to the fold depending on the fit.