Basically all you need for this is good balance and a little bravery. Start small. Go off small drop-offs before going to the 6-inch curbs. Then, just ride off the curb and try to keep riding! You may find that hanging onto a person is also helpful in learning.
We find that it helps a lot to hang onto the seat until you hit the ground, and you are less likely to lose control. It's very handy to be able to jump down curbs without dismounting.
Some people can go off huge drop-offs, like the legendary guy who went off a FIVE-foot drop off. Of course, you have to be careful not to bend your rim!
Jumping UP a curb is another story! Some people (not us) can go up and down a flight of stairs. Amazing!