For those who asked, this is how you do step 4 of the Circle Skirt Tutorial.
a. Fold the fabric.
b. Trace points with the end of your tape measure serving as anchor. Do it until you reach the other side.
c. Draw a curve.
And you have your waistline! Yes, this is circular but since it will be on the bias, you will be able to sew it to the waistband or the top part of the dress with no problems.
Do the same for drawing the hem, using the corner as reference point.