Scanning all of the enemy types is easy enough to do, after defeating them in combat just press down on the d-pad when BD-1 indicates for the scan, however some of the enemy will be automatically added into the Tactical Guide such as bosses.
You can track what enemies you have scanned via the Tactical Guide section of the pause menu. The enemies are split into 6 sections to make it easier to work out where they can be found within the game.
The scannable enemies are as follows:
Empire Scout Trooper
Scout Trooper Commander
Stormtrooper
Stormtrooper Commander
AT-ST
Security Droid
Flametrooper
Heavy Assault Trooper
Probe Droid
Rocket Launcher Trooper
Electrobaton Purge Trooper
Electrostaff Purge Trooper
Purge Trooper Commander
Electrohammer Purge Trooper
Bogano Bog Rat
Oggdo
Splox
Oggdo Bogdo
Zeffo Scazz
Phillak
Jotaz
Skungus
Tomb Guardian
Rabid Jotaz
Dathomir Bane Back Spider
Lesser Nydak
Nightbrother Warrior
Nightbrother Archer
Undead Nightsister
Nydak Alpha
Kashyyyk Flame Beetle
Wyyyschokk
Jaw Plant
Slyyyg
Saava
Mykal
Albino Wyyyschokk
Haxion Brood Haxion Brood Bounty Hunter
Haxion Brood Commando
Haxion Brood Bounty Droid