Found this free tutorial today and decided to make some for my 3 year old. He has a 19 inch waist, so I cut the waist band 3 inches by 15.5 inches. The gusset wasn't my favorite, but I love how easy it was, and that I can repurpose all those old shirts of my boys I was just planning on giving away. :)
By the way, I think if you just did the steps until you finished the waistband, you could have a cute skirt for a girl.