if that's the project you want to start with you can do this:
cut out one-quarter of the circle you want to make. it will be a 90* angle. fold it into as many pieces as you like. if you choose to fold it into 4 pieces, for example, then 16 pieces would make the entire circle. if you fold it into ten pieces, then you need 40 pieces for the entire circle. what you do for the one-quarter, you have to multiply by 4 to get the whole amount.
when you cut the tree skirt to it's full size, you'll know how wide each wedge has to be at the widest point, depending on how many wedges you use.
when you decide on the number of wedges you want, cut out that paper wedge. now you have a pattern for the correct size wedge. trace around it, adding the seam allowance. remember to cut the right number. at one point in the skirt, do not attach the two wedges, but bind it as you would a quilt, or leave it open and turn it right side out, envelope style, as you would a quilt. you might want ties to close it up. btw, this is like a dresden plate, so you can scallop the edges or make points in the same manner.
if you think you aren't going to use one of those rulers more than once or twice, this is very easy. those rulers are pricey.