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1. Cut the oversized button up an inch or two below your natural waist. Take the top half and cut off the sleeves. Then try it on inside out, mark where it needs to be taken at the sides and sew the new side seams so it is nice and fitted. Cut the arm opening to hit a half inch from where your shoulder seam will be. Do the same with the sleeves, make sure the openings of the sleeve and the top half of the shirt are the same length. You may need to take a pocket off if there is one.
2. Take the bottom half and gather the cut end to be the size of your waist.
3. Re-attach the sleeves by turning the top half inside out and the sleeve right side out (right sides together) and pin the openings.
4. This is what it will look like attached.
5. Attach the gathered bottom half and the top half right sides together. Make sure the middle button parts line up perfectly.
6. Serge the raw edge to prevent fraying, you can use a zig zag stitch or what you prefer on a sewing machine instead.
7. Top stitch the finished inside edge to just above the gathered seam to make it look nice and finished. One thing I would have added was a button or hook & eye right where the seams meet up because it easily pops open.