I got into it as a kid. Once I understood that it's a matter of visualization, it was pretty easy.
Half turn, full turn, 1 1/2 turns, two turns.... that's as far as I got. The trick (for me at least) is to see what you want the knife to do, concentrating on the point, all the way to the target. If you have this firmly in mind at the instant of release, you've got it made. If you lose your concentration, you'll miss.
The longer knives, 10-12" at least, are easier to learn with. I have a bush knife that I cut down from a machete that's 14" long. The point of the Bowie-shaped blade is on the centerline of the handle. I can hit with that one out to 15' or so.
The usual 7" "throwing" knives are about useless. If you want to learn, get a big, heavy, cheap knife and practice on trees. Hold the knife by the blade and visualize the knife turning over and the point entering the target. Start at 8 or 9 feet. The bigger the step and the faster the throw, the slower the turn.
You'll figure it out. It's kinda fun.
John
Cape Canaveral