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!

Materials
fabric for underskirt (~2 yards)
tulle (4-10 yards; more will make a fluffier skirt, so go with more)
elastic
fabric tape measure
scissors + pencil
sewing machine

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.

Step 2: Cut and Sew

Now cut out your inner circle, tulle & underskirt fabric.

For the outside circle, I wanted the tulle to be a little longer than the underskirt. With the tulle and underskirt still pinned at the inside circle, cut out the outside circle of your underskirt. Then go back and cut the tulle a few inches longer than the underskirt.

You now have multiple circles made out of tulle and fabric!

Unpin the tulle and fabric. Lay the tulle out on the floor unfolded, in a complete circle, then put the solid underskirt in the middle, so the cut circles line up, and pin this line.

Sew the tulle and lining together, without the elastic.

Step 3: Try On, Make Adjustments, Add Elastic

Now you have a circle skirt of tulle-and-lining, with no elastic waist! This is when you should STOP and TRY IT ON. Mine ended up being a little tighter than I needed, so I had to cut some seam and make the hole wider so it would fit over my hips. If you do make adjustments, make sure you end up by having a complete seam line around the tulle and lining for the inner waist.

Now you're going to sew your elastic. At this point the inside circle of your waistband may be longer than your elastic, and that is okay and how it should be!

I did not pin the elastic for this, because as you sew you're going to stretch the elastic.

Leave a half inch or so of overhang on your elastic, and start sewing to the inside of your circle. Stop! You now have the elastic initially sewn in place. Now you're going to stretch the elastic over the tulle as you sew. Keep sewing the elastic around the inside of your circle, keeping it stretched the whole time.

When you're done with this seam, you'll have elastic with open ends. Safety pin them together and try it on!

Now sew the end pieces of elastic together. I did this by folding one end under and putting it on top of the other end, so there would be a smooth seam inside and out.

Step 4: Make Adjustments and Wear!

Congratulations, you have a tulle skirt! That was easy, right?

One final step before wearing is trimming your skirt. My tulle was longer than my underskirt and also quite jagged, so I used scissors, while wearing it, to trim up the tulle so it was even all around.

Now you know how to make a tulle circle skirt, and from here you can start doing the fun stuff. Add in several layers of tulle! Make them different colors! Wear it with a cape, to the grocery store! Sew a ribbon over your elastic waistband and tie a cute bow on the side! Make a longer one, or a shorter one! Wear under other skirts and dresses for poofiness or just as is for fun.