For onions specifically:

Sharpen your knife and make French onion soup.

You’ll cut so many that you’ll figure it out.

For everything else: pinch grip and crab your other hand. The pinch grip is where you rely on a pinch between thumb and forefinger on the blade just in front of the handle to grab the knife. It’s the choke up of holding a knife and will make you much safer and give more control. Crabbing your other hand is where you curl your fingers up like ginger roots instead of letting them extend out like little baby carrots. It will keep you from being hurt when something goes wrong and allow you to go much faster because you’re not having to slow down to avoid cutting yourself.

Keep your knife sharp, remember that you cut by running the blade along something rather than pressing into it, and keep your fingers out the way by doing the claw with the other hand and keeping your grip firm. Then just practise!