I've cut down a button-down shirt...and while it may not be fancy, it works.

1. Do you want to keep the collar? If not, carefully cut it off. Lay the shirt flat with the front buttoned up and mark out a curved neckline so that a button / buttonhole is at the bottom of your curve - or at the top of where the buttons are to start - whichever makes more sense. If the shirt is loose, sew a small casing all along the edge and work in a small elastic (like 1/4").

2. Cut straight across the back to each side seam however short you want the shirt. This next part creates "tails" that can tie in the front. Cut in an angle from your side seams down to the front edge. You may need to try it on and adjust your angle cut to fit so it's not too blousy in the front.

I hope I explained enough to get you going.

there are also some really cute fitted flannel shirts at stores like forever 21 and pacific sunwear. might make it easier to fit it to you without having to sew alot.

Pictures of the finished project in my profile!