Here is a rough (VERY rough and not to scale) diagram of the skirt. For the record, I am 5'5" tall, so YMMV (your mileage may vary) if you are a different height.
And here is how I created it:
1. I cut a rectangle 66 1/2 by 17 inches. This is the piece that extends from side to side and contains the waistband.
2. Locate the precise center point of the rectangle - this is the CF/CB point. If you fold the rectangle in half the long ways, the fold is the side "seam" (it's not actually a seam, but you can think of it that way). Cut a straight line along that fold, starting at the middle, or CF/CB. If you want to make the waist the size of your exact waist, you will have to add a vertical slit (shown on the diagram). This is how my original skirt is made. If you want to make the skirt larger than your waist (because you are using elastic), you can omit the vertical slit. To calculate the length of the slit, divide your waist opening by two. So, if your waist is 30 inches, you might want to add another 10 or so inches for gathering (and so you can get in and out of the skirt). Half of 40" is 20". So, you will cut 20" across the fold, centered at the middle point. (I start at the middle, cut 10" to one edge along the fold, then return to the center point, and cut 10" in the other direction for a total of 20." This ensures that the skirt is centered on the waist.)
3. Cut your waistband. Mine was 3 1/4 inches wide (calculated using a 1/2" S/A for attaching the waistband and using 1" elastic) and I cut it extra long and trimmed it to fit as I went along. I made a four-part waistband because I wanted the elastic confined to the sides. I won't go into that detail here, but there are patterns that use this technique.
4. Attach the waistband to the slit opening. This might sound tricky, but if you've sewn a continuous placket into a cut slit on a sleeve hem, you've done this before. In this case, you just follow the process at both ends of the slit.
(If you aren't sure how to proceed, google the subject and you can find a tutorial.)
Here is a view of my skirt with the waistband attached. The puckers are caused by the elastic which has already been inserted into the waistband.
And here it is again, folded along the side "seam". You can see I channel stitched the portion of the waistband that has no elastic. I did this to give it more body. I didn't use interfacing because a) I didn't have any and b) I don't want the waistband to be particularly stiff. The channel stitching was sufficient (for me).
5. Attach any trim, if desired. This is where I attached the fabric selvedge. You can actually see that in the picture above where the rectangle is laid out flat.
6. Attach an inseam pocket, if desired. I held the skirt up to my body and decided on the pocket location. If you've ever sewn an inseam pocket into a pair of pants or a jacket, it's the same concept, except the pocket is a rectangle shape. Here is a Inseam Pocket tutorial, if you need help.
6. Cut two skirt bottom pieces. Mine were 66 1/2" (the width of the skirt) by 24". Sew one to the front and one to the back.
7. Try on the skirt. The corners of the square are most likely "dusting" the floor. Mark where you want to cut them off.
8, Take off the skirt and lay it out flat. Draw a curve on one corner to shorten it to the point you have marked. Cut off the excess fabric.
9. Use the trimmed corner piece that you cut off as a "template" and round off the remaining three corners.
10. Hem the skirt. (I folded the edge 1/4" twice and stitched by machine.) The hem is uneven by design. :)
11. Wear and enjoy. I'd love to hear your experiences if you make a skirt using these instructions.
Note, if you don't want the inseam pocket or any sort of decoration, you can make this skirt as one big square.