Starting at the toe and working towards the instep, I created a “ladder” to go around the back of the heel.
Filling in the back was accomplished by binding 3 into 1 at the corners, and 1-1 for the rest, until the opening at the bottom was a couple inches across.
One more row of nålbinding firmed up the top edge.
When the bottom opening was a couple inches across I decreased to make a flat circular heel.
Another pair of happy feet!