For my daughter' skirt I used double her waist measurement plus 1" allowance for the zipper. Each pleat took 6" (3" on the inside and 3" on the outside of the skirt). Her waist is about 24" and I added 1" allowance so 25". I made 8 pleats (8x3"=24") plus 1" seam allowance gives us another 25". Total was 50". What I did was cut a long strip of fabric with the width matching the desired length. Then I added half the zipper allowance at one end, started to do the box pleats, stopped when I met Ellie's waist measurement and added the other half of the zipper allowance.
As for the length, that is up to you. I went with a knee length and subtracted the ruffle hem which is 3" including allowance.
Grab your seam gauge and starting marking every 1 1/2", leaving 1/2" allowance at start and finish to accommodate the zipper.
Grab the first and third mark and meet them, pushing the second mark inside until it meets the third mark on the back of the fabric. Pin.
Meet the fourth and sixth mark, pushing the fifth mark inside until it meets the third mark on the back of the fabric. Pin.
Repeat in the same manner for all pleats.
Sew the top of the pleats with 1/4" seam allowance.
Press just the first couple of inches of the pleats with a pressing iron on steam.
Cut a long strip of contrast fabric, double in length than the hem of the skirt and as wide as you like (I used a 3" wide strip).
Make a simple hem of about 1".
Gather the strip into a ruffle along the opposite side.
Attach the ruffled strip to the hem of the skirt. Place the skirt right side out and the ruffle wrong side out, raw edges together and sew along the raw edges with 1/4" allowance.
Finish the seam with a serger or a zig zag stitch.
Fold the hem down and press with iron on steam.
If using a regular zipper, fold the skirt in half width-wise, wrong side out and sew the ends with 1/2" allowance. This will be the back seam of your skirt.
Attach the zipper to the back seam. My zipper happened to fall right in the middle of a pleat which I think was wonderful.
To make the waistband, cut a strip of fabric with the waist measurement plus 1/2" allowance for folding the ends in. The strip should be about 4 1/2" wide.
Fold the strip in half and press.
Push the ends in 1/4" and press.
Fold each half of the strip in and press.
Attach the waistband to the skirt by simply sandwiching the top between the waistband and sewing 1/8" away from the open edge of the waistband.
Give the skirt a last press and you're done! Enjoy your box pleated skirt!