Here is a tulle skirt I made, because I'm 23 now and I need a tulle skirt for everyday looking-fancy needs as well as you can wear it under other skirts to make them poof out adorably. In case you need one, here is how I made it!
fabric for underskirt (~2 yards)
tulle (4-10 yards; more will make a fluffier skirt, so go with more)
fabric tape measure
scissors + pencil
Before you start, WASH AND IRON YOUR UNDERSKIRT FABRIC. You do not really need to wash tulle (or iron it, obviously) but I've heard it softens up if you do. Up to you! I didn't wash my tulle first.
Step 1: Measuring
Measure around your waist where you want the skirt to be, using the fabric tape measure. For me, this was 30", so I then cut a piece of elastic to be 30" long.
You're making a circle skirt, so it's math time! (It's easy I promise)
(your number) / 3.14 = (number 2)
(number 2) / 2 = (number 3)
so 30" waist / 3.14 = ~9 inch diameter = 4.5 inch radius.
This is also known as figuring out the radius of a circle.
Fold your underskirt fabric in half and measure how long it is, then mark the middle. From the middle, take your radius and draw a half-circle on the fold line. You're cutting along the fold line so you will make two cuts--the inside circle and the outside circle--and have a full circle skirt.
Measure out from your inner circle to the outer circle--this will be the length of your skirt. My skirt is 15" long, so you can look at the first picture and use that for reference as a just-above-the-knee skirt (or shorter, depending on how high you wear it).
Draw the line for your outer circle. You should now have a piece of fabric, folded in half, with two semicircles drawn on it.
Now take your tulle, also folded in half, and place on top of the underskirt fabric. Put as many layers of tulle over this as you have, and don't worry about if it's too long or short or overhangs at one end.
You're going to cut through all your fabric at the time, so you'll be making three to six circle skirts (from three to six different pieces of fabric) at the same time! Much easier than measuring and re-drawing for all the tulle.
Pin your fabric near the line of your inside circle. This will stop it from moving around when you cut it.