Take Your Flannel Shirt and Cut the Sleeves. (I forgot to illistrate this step in the image, so just disregard the sleeves) Mark just above the pockets and the width of the pockets then draw a diagnal line from each side to the corner of the shirt. The red line on the image demonstrates how the area should look. Cut out this shape on both side of the shirt.
Grab the sleeves from before and cut out a triangle on each. Make sure it is the same length as the previous shape you cut out.
Now you have to sew the pieces together. Take the shirt pieces and the sleeve pieces and line them up in alternating order (ie: front of shirt, sleeve, back of shirt, sleeve). Make sure the smaller parts of the triangles are the top of the skirt and the wider edge is the bottom. Stitch these all together.
Cut out a waistband from the excess fabric on the shirt and line the button on the waist band to the buttons on the skirt. Attach the waistband!