Step 5: Aiming up and down
This gets trickier...
Turning your wrist is part of it, the other bit is the fingers releasing the card.
The angle of your wrist decides the angle the card flies away, turn upwards for up, down for down, however it's really small movements, start with small angles and learn to control it finely. You'll learn that there are limits to the angles a card can fly at to start with, usually this is due to the throw, it's the same issue as frisbees...
The other deciding factor are your fingers on release, if they're unevenly extended it'll impart a different, curving angle up or down. The best way to master this is paying attention to the feeling of the card leaving, it's something that is really tricky to consciously adjust, more learned intuitively.
For the actual learning the best way to do it is a big room and things of various heights.
A stepladder would be perfect, learn to throw the same way as before, starting with between two rungs in the middle and using them as compartments, start 10 feet away and slowly increase the distance until you can get the card through each one easily.
As a challenge try hitting the rungs themselves.
Bookshelves also make good targets for this.