You will want to land with your weight on the pedals, which reduces pain and lets your legs act as shock absorbers. When you land, you pause for a quarter to a half of a second before pedaling again to get your bearings. Then back up the wheel carefully, mount the uni, and go down the curb. This method ensures that the pedals are horizontal for landing, which is the easiest way to do it. After confidence and skill builds, it won't matter where the pedals are when going down curbs.
Some people prefer to jump off of the curb, rather than dropping off.
The advantage is that you will land with the pedals in a good position and you can control the jump. Lower the seat an inch or two. Once you can do the trick comfortably, you can raise it again. Make sure you practice landing with either foot forward. This is very important if you want to become good and able to go off curbs in all circumstances.