First find a really big men's dress shirt that you like the pattern of, the bigger the better. Here is what I started with:
Set those pieces aside, time to go back to the top section. Fold under the edges and sew up the arm holes. If you need to bring it in, now is the time. My seam ripper is acting as an arrow, this is where I sewed to make the arm hole shorter and bring it in across the front.
Next we are going to attach the largest sections from the sleeves. Don't worry about sewing them together just yet. Even them up across the front and the back.
Now we reattach the bottom portion of the shirt. I missed a few pictures here, but here's the lowdown. Line up the buttons, the back centers and the side seams and pin those places. Since your skirt piece is probably wider than the top portion, add a few pleats by pinning them into place evenly across the front and back.
Shirr several rows on both sides. I originally started in the middle of the panel but decided to shirr the whole thing, making it lay nice and flat across the front of me, not adding any extra width to my middle!
... or cut it off and have it be more of a tunic length?
Peace be with you,