Inspiration - Elizabeth and James
Sew new side seams up to 2" in from the original side seams depending on how big your shirt is. Be sure the seam continues past the arm hole seam and into the arm.
Pin your pleats in place.
Mark button placement
Line edge of button with edge of pleat. Sew on your buttons. Be sure to sew buttons all the way through to inside of shirt. I also added pleats to the sleeves, tacked with X stitches.