I learned knife skills from (a) practice and (b) watching videos.
(a) was far more practical than (b), in that I sliced various parts of my digits off and even managed to cut right through the ball of my thumb with a 12" chef's knife. Learning tip? DON'T do that again.
The classic method for slicing veg, onions, preparing brunoise, etc, is the "bear claw" - ie, you hold what you're cutting with your hand in the form of a claw, and use your knuckles as a guide for the knife, always keeping the point of the knife on the cutting board.
But you knew that already, right?
Can't tell you how many knuckles of mine got involuntary plastic surgery...