Start by taking the measurment around the widest part of your bottom half (i.e. your bottom).
Also measure around your waist.
Cut two rectangles to the width of those measurments and the length of your choice. (double the length on your waist rectangle.)
Fold both in half and sew down .
You should then have two pieces that look like this.
Next cut two long strips the length of your skirt by about an inch.
Fold in half and sew. Turn right-sides out using a safety pin or whatever method you prefer.
Cut your straps in half so you have a total of 4.
Next you're going to fold your waist band in half, wrong sides together. Pin it to the top of your skirt matching your seams.
This next part may sound a little confusing it isn't really once you do it.
You're going to pin your strings on the sides of your skirt so mark where the sides are.
Next pin one of your strings on what will become the inside of your skirt and the other sandwitched between your waist band and your skirt.
Repeat on the other side.
If you are using a serger, I suggest doing a stay stitch with your sewing machine first. Use a long stitch in a contrasting color so it's easier for you to remove later. Stretch your waste band as you're sewing so that it fits to your skirt.
All you have to do now is hem and then you can wear!