The fabric should have the double of your waist measurement by the lenght of your skirt.(you will need a bit more of fabris for the waist band)

Now mark the middle of the fabric.

Start doing folds looking to the center.

When you finish doing the folds you sholud have something like this.

Now take the waist band,pin it like this and flip the skirt.

Turn also the waist band and sew it like in the picture.

start sewing both ends together and add the zip at the end.

You will also need to add a button to the waist band.

and that's all! I hope you like it ^.^