Use the blade of a pocket knife to shave a bit of the magnesium off the back into a pile on something study, put some tinder above it and off to one side, set the corner of the metal match on the study item at about a 45% angle with the flint side up and pointing down into the tinder.
Use the back/flat side of the blade and run it quickly down the flint and the sparks will jump down into the magnesium setting off the tinder. The key is having good tinder - the rest is easy.
Get one and play with it in your back yard. The first season of Survivor I took my little girls (about 7 at the time) out in the back yard and taught them how to do it. Had little burnt spots all over the yard. Momma was a little POed when she got home. They think it’s a neat trick.
I am the clumsiest person in the world, really.
If I got one of those and practiced in my backyard, we would be looking at the Small Town Americana version of the Great Chicago Fire.