Start with your fabric. I have made this skirt for a 4 year old. I like to use the basic sizing found Here.
I like to start with my fabric 15 by 45 inches. For a good length measurement. Take your finished length you would like the skirt and add 2 1/2 inches. This with give you one inch for the Hem and 1 1/2 inch for your waistband.
Sew your Two short ends together. These will also be your selvage edges.
If you have a serger you can serge your 2 long raw edge.
If you have a serger you can serge your 2 raw edge.