This procedure is so much fun and so quick it is over all too soon. As usual preparation is key. First fit an open, short-toed, button sewing foot to the presser bar.
Next raise the throat plate (which is, on the 401, the equivalent of dropping the feed dogs - trust Singer to be different!)
The stitch selector must be set at AL and the red lever at position 1.
With the work clamped down by the presser foot and the needle aligned with a hole on the left side of the button take one stitch (turning the balance wheel by hand) and stop with the needle just above the foot.
Then move the red lever to position 4 and take six zigzag stitches ending with the needle back on the left side.
Move the red lever back to position 1 and take three stitches to secure the button. Remove the work from the machine and trim the loose threads. It's that easy. It takes seconds. I kid you not!