First start with a t-shirt.
Cut off a band in the desired width. Honestly I just eye balled my cutting but you could get technical and use a ruler and stuff. Leave the bottom just how it is because this will be the perfect hem for your skirt.
Serge around the cut edge. This isn't necessary I guess but gives it a nice finish. You could zig zag with your regular sewing machine if you don't have a serger.
How it looks once serged.
Next fold under and press the desired amount to form a casing for the elastic. I made my casing just slightly larger than the width of my elastic which was 3/4". Sew along the bottom edge of the casing leaving an opening about an inch and a 1/2 wide to thread the elastic through. I measured a pair of small fry's pants to get the length of elastic I would need. Trying to measure her waist would be like trying to measure the waist of a greased pig. Put a pin in one end of the elastic and thread it through your casing. When you have both ends at the opening overlap them and sew them together. I sew back and forth about 5 times for extra stability.