Pull off the outer layers, the brown papery bits.
Look at what you have underneath, if it's still brown, or yellow, dried, tough, soggy, mouldy - keep peeling.
Peel the onions down to firm white flesh, which is thick and juicy-looking. If you've picked good onions you shouldn't have a lot of peeling to do.
Cut the top off back to where the onion looks good to eat (as above)
Cut the root base off flush to the rest of the onion, but no more. Don't take a slice
off the bottom, because this bit will hold the rest together while you're cutting it.
In the video you see me cutting towards my thumb. Notice that this is a one handed action, I cannot put much force on the knife and I have a lot of control over the blade. It's hard to cut yourself in this way.