Im going to work you through what to do as well as what not to.
First, how long do you want your skirt to be?
I decided 42 inches to give me plenty of room for seam allowances.
Then you figure out the waist/hips measurement. To do this, use C=2πr
So, if you are like me and want a 40 inch waist your half circle the math would look something like this:
C = 2πr
2(40) = 2πr
40/π = r
r is about 12.7 inches.
DO NOT: Think that your waist measurement is equal to the circumference, it is HALF!!! (Half circle has to be the FULL distance around your waist). When I cut out my material, I had a 20 inch waist because I thought 40 was the correct value for the circumference:
Guess who doesn't have a 20 inch waist!
So, now you have the radius of the waist,correctly calculated let's hope, and the length of you skirt. Add them.
In this case, 12.7 + 42 is 55 inches.
Youll want to measure this 55 inches from the folded corner of your fabric (assuming that it is folded in half) and cut out the 12.7 inch top.
PS: I was able to use the excess material to add rectangular panels on each side. I can wear the skirt now, but it isnt as twirl-able as a proper half circle skirt :(