Legion Name: The Legion's name. This can be its original name as described by the Emperor, or a name given to it by its Primarch after their recovery by the Imperium. Or you could list both. The Warmaster's Legion will be called the "Sons/Daughters/Children of _______" and then later, the iconic "Black Legion".
Legion Number: The Legion's number, in Roman numerals up to twenty (I, II, III, IV, and so on up to XX). Obviously, only one Legion per number, on a first come first served basis.
Legion Strength: How many members the Legion possesses. This can be anywhere from "merely" several thousand Astartes, to substantially greater than one hundred thousand units, and will determine the overall strength of the Legion in terms of sheer brute force, though how skilled its members are are is another matter entirely.
Armor Appearance: Using this SM armor painter, make the color scheme of your space marines, they haven't turned full chaos yet so they won't be having any of the spikey bits and stuff. I'm not going to be too much of a stickler for exact marks of armor and such, but you should not be having like Mark 8 power armor.
Warcry: What members of the Legion yell when going into battle, e.g. "Courage And Honour!", "Repent! For tomorrow you die!". A Legion may possess more than one warcry.
Important Figures: Besides your Primarch, who else is a "major player" in your Legion? This could be anyone from company commanders to techmarines or medicae or whatever. If you have a splinter force of your Legion who did not follow with your Legion's overall choice on what side to be on in the Schism in the future, this is where you would put them.
Tactics/Roles: Basically how does your Legion fight and what job/role do they normally occupy? Are they siege specialist? Melee champions? Ranged marksmen? Or something else? What kind of vehicles do they have and do they use orbital assists?
Traits/Ideology: How does your legion think and appear? Is it due to the Imperium/Chaos influence, its gene-seed, its Primarch or its recruits/homeworld? Essentially, what is the "personality" of your legion and what kind of look they take on that could take cues from real world cultures. Traditions can also be listed here.