this sweater from Banana Republic:
I started by cutting off a large strip from the bottom as well as the thick strips of fabrics with the buttons and the button holes.
From the front of the sweater, I only cut off enough of the fabric to remove all buttons and buttonholes. That way, I still have a nice thick collar like I wanted. (This picture shows where I stopped cutting.)
I ended up with three strips of removed fabric:
To keep from fraying, I added bias tape to the cut edges, similar to the tutorial here.