When I first started posting on SWN I was asked how I made the twirly skirts for my dd. So I had written some directions and Mary posted them in her articles. I am posting them here so they don't get lost. I am the author, so I don't think I need permission, right??
I will add pictures of the process to it later, I've got some pics of the process, but I'm going to make a new one soon and hope to get better shots:D A few more pics of the ones I've made in the past are at the end.
*I've written these directions for anyone to use, You can use the finished product any way you'd like. The written directions are not copyrighted, however, if you are going to copy it to another site, please link back to here or give me credit! I've seen these exact directions posted elsewhere with the person taking credit for them
**Measure your child’s waist. Example 20"
*Decide how long you'd like the skirt to be: ie: 15"
*Decide how many tiers you would like: 3
*Divide your length by the tiers and add seam allowances:
15 divided by 3 = 5.
Example:
If using 5/8 seam allowance, add seam allowances of 5/8" remembering that the top piece you will need to add enough to allow for a casing for the elastic size you choose + the 5/8", so 5+ 5/8 + 1"= 6 and 5/8". On the middle piece it will be 5/8" on both top and bottom so you add 1 1/4". On the bottom, allow for whatever hem you like to do, I like to add 5/8" and do a narrow hem, so I've added 1 1/4.
*You will be cutting and/or piecing fabric into strips that are 1 1/2-2 times the width of the strip above it.
Example:
Waist size 20", top tier is 30-40" wide
Middle tier is 45-80"
Bottom tier is 67-120"
I usually round up my numbers depending on the fabric. You can add seam allowance to this too.
*I usually make the skirt starting at the bottom/longest strip. It seems to be easier than starting at the top. Sew the strip ends together right sides tog. Mark the quarter marks to help ensure even gathering. Gather the top edge (I zig zag over dental floss to make the gathers). Then sew to the next strip right sides together, press, trim, and finish the edge. You can top stitch if you want. Repeat with each strip adding it to the top of the previous one.
*Make a casing at the top/waist for your elastic by finishing the raw edge and folding over and stitching, or press the raw edge under and edgestitch leaving an opening to insert your elastic.
*Cut elastic to fit comfortably around waist, insert into casing and close the opening. Adjust fullness on the elastic.
If you want to add this skirt to a t-shirt then don't make the casing and don't allow extra for the elastic, just the regular seam allowance. Just attach it to the t-shirt as you did each strip right sides together using a ballpoint needle.
Hope that is what you were looking for! If you have any questions I'd be happy to help!
) , so sometimes it is not necessarily exact. As long as you have the waist a comfortable fit and each tier below is wider by atleast 1.5 times, you'll get a pretty good result. Also, if you want more "twirl", make sure you do atleast 2x's the circumference. In the pictures I posted, they are at least 2x's. The only one that was figured at 1.5x's is the Tshirt dress, as you can see there isn't as much fullness. You can also make the tiers different measurements, in one of the skirts, there was only 3 tiers with the middle one longer than the rest. So you can really do whatever you want kwim? So don't worry too much about the math 
I can't go fabric shopping because my dh will have my head if I don't start using up some of the fabric I have.