Measure around your child's hips at the widest point. Add a few centimeters to this measurement and write the number down.
Spread out your fabric and mark the horizontal edge with the hip measurement. Cut off the excess fabric.
Decide how long your child's underskirt will need to be and add a few centimeters to the measurement. The extra fabric will become the hem and the waist of the underskirt. Mark the measurement along the vertical edge of the fabric and cut off the excess material.
Turn the fabric wrong side up, and fold the horizontal edge over a few centimeters. Iron the folded edge and pin the fabric at intervals. Sew along the fold.
Fold the skirt in half with the wrong side facing up. Sew the edges together leaving a small border. Leave the top fold open.
Attach a safety pin to a length of elastic. Thread the safety pin through the top fold of the skirt all the way around the waist. Pull the elastic until the waist is gathered and sew the ends of the elastic in place. Finish sewing the top edges of the skirt together.
With the material still wrong side up, fold the bottom edge of the skirt up by several centimeters and sew a hem.
Trim off stray threads and the excess elastic. Turn the skirt right side out and iron out any wrinkles.