NOTE:
I realize this is not true for every machine. If you want to be sure, look to be see that the curved part of the needle goes next to the hook. That is the sure fire way to have the needle in right on every machine. The info about the flat part of the shank going toward the shaft should work for your Singer 15s and 221s and 201s and many others - double check which side the hook goes compared to the needle.