So, I went to Goodwill for a shirt and just did my own thing.
Here's the tutorial!
Step 1: With a seam ripper, remove the pocket/s. Save these to reattach later, or don't. Lay out the shirt as flat as you can and cut straight across the bottom, so that all of the curved hem is removed, and straight across the middle under the sleeves. You'll see in my photos that I cut a little higher on the top, cutting in around the arm holes. This is because I am a tall girl and I wanted this skirt to pass the fingertip rule(hem is below my fingertips while standing straight with my arms extended down, for those of you who didn't attend public high school in the early 2000s. :)) so I could wear it to work. Unfortunately, this took away some of the width of the skirt, so it wasn't as full and gathered at the top as I would have liked, but c'est la vie. If you don't mind a shorter skirt, cut straight across under the arms.
Step 2: Measure the circumference of your waist and cut strips of flannel from the upper back or sleeves that are 3.5 inches wide and your-circumference-plus-the-width-of-your-button-placket-plus-1/2inch long. (Example: my waist circumference = 30inches, my button placket = 1.5 inches, so my waistband piece should be 30+1.5+0.5=32 inches long) You may have to cut two strips and sew them together in the middle to make one long enough. This will make your waistband.
Step 3: If you cut high around the sleeves, turn the tube of the skirt inside out and pin two lines down each side from the top corners to the bottom corners of the same side. If you just cut straight across under the arms, you don't need to do this step.
** From here I'm going to deviate from what I actually did and tell you how I should have done it. Hindsight, as they say, is 20/20 and as I was writing this I saw a much simpler way of finishing this skirt.
Step 5: To create the bottom hem of the skirt, unbutton the tube of the skirt and lay out right-side down. Fold up and press 1/4 inch of the bottom edge, pin, and sew with a straight seam. To save length, I did not fold it twice.
After pinning, the ends should look like this:
Step 7: Sew a straight seam 1/8-1/4 inch from the bottom of the waistband.
Step 8: Button up the skirt again, right-side out, and sew one seam up the left side of the button placket until about 4 inches from the top. The last four inches will allow the skirt to go over you hips when you put it on.