Putting the Galaxy Note 2 in download mode is similar to Note 1. 
Just press the vol down + power button + home button simultaneously. A warning screen will come up. Just press the Volume Up key to continue booting your Note 2 in download mode.
The mode is used to flash custom firmwares, clock work mod recoveries, radios and so on. Make sure you know what you are doing as you can easily screw things up in download mode by flashing a wrong ROM.