The yardage of tulle you will need depends on both the height of the costume wearer and, of course, the width of the tulle you are buying. If you order through the Amazon links below (which is sold on 6-inch-wide spools), buy one spool of each color, and you'll have some left over. You will cut the tulle lengthwise into 3 strips of 2 inches each, as long as you need them. (See instructions.) If you're buying at a fabric store, the bolts are likely to be 54" wide, which should be long enough to cut the strips crosswise unless the wearer is very tall or wants a very long skirt. In that case, you can buy three yards of royal blue and two yards of light blue.