Hitting uses an approach. You can take big steps, but not leaps. They are all steps, especially that last one. Gathering is bending at the knees in preparation to jump. Use your arms to help throw yourself upwards.
Footwork is realy tricky, especially for middle blockers.
The most important things here are to plant and gather before jumping and to jump straight up. Planting means you have both feet in place and still before you gather. The biggest mistake is not jumping straight up. Not only does it waste energy, but you jump into the net, or the player next to you. The flying leap to block is especially poor form because more often than not you will block out of the court and throw away the point.