This is easier than the user-friendly methods (sorry DietPi). I have it running happily alongside Plex and RetroPie, and it wasn't painful.
If you're installing Emby on top of an existing system that you like working with, back that up. Otherwise, start by flashing your card with the most recent Raspbian image. Set that up to your liking.
You also of course need to have a drive or drives ready to keep media files on. Please don't try to do it on the Pi's SD card. My drive simply has folders named "Music," "TV Shows," and "Movies." It was alreadly organized for Plex. Make sure the drive is plugged in and mounted.