I have done that, what I'm going to do next?
Actually, to correct Shadow#1 above you will also need the standard version alongside mode3. Like he said, get any banner version.
For most games, the normal CIA will suffice. Just load it up, choose 3.6, then wait for NTR to launch. That one will exit to the home menu where you can then proceed to launch your game.
Some games, however, require that O3DS models reboot into an "extended memory mode", or mode 3, which turns off certain services that "aren't essential" giving the game much more RAM to allocate and decreasing CPU load leading to better performance. This "soft reboot" is what prevents the standard BootNTR from working with these games on O3DS. The best way to identify these is the long reboot time when loading or exiting a game, though another method is to try to launch the now-dead Miiverse where you will be greeted with an error message before the app even starts. Well-known titles that do this include Super Smash Bros., Pokemon (Ultra) Sun and Moon and Monster Hunter 4U. (correct me if any of these details are wrong)
New 3DS consoles already have increased RAM and an upgraded CPU, so those games will run as if they were N3DS enhanced instead, no reboot required, removing the need for a mode 3 version.
For these games, launch the mode3 CIA, wait for it to finish loading, then go to the home menu (do NOT quit as the console will reboot into standard mode and NTR will quit) and load up the title you want to load directly. As the console is already in extended memory mode and does not need to reboot, the game will load directly and NTR will be usable.