Dear Jack,

Overcasting is a little different, and needs to be done with a thin
thread. Come up from the back on the machine stitched line; from the
front, go to the back again right next to the first hole from the
needle. The thread will straddle the edge. Go all the way around,
then switch to the buttonhole twist to make the finished buttonhole.

The coat looks great, and the robin is a delight!

Teri