Sewing Outer Edges Non-Serger Method:
If you don’t have a serger, then you’ll want to set your sewing machine on the zig-zag stitch setting. Zig-Zag stitch from the arm edge, down the bottom of the cape, on both sides. Serger Method:
If you have access to a serger, then you can use that and just serge from arm edge all the way to the bottom, and then repeat on the other side as well.
At this point you’ll have an opening at the bottom, the neck hole and the hand holes.
Here is a close up of the serger stitch by the arm and the side of the cape.