Simple enough, just give a basic history and details and any special rules about your space marine chapter.
Steel Dragoons
Colors: Grey/Silver
Banner: Half MkIV helmet, half skull
Motto: On a one way ticket to midnight
Founding Chapter: Iron Hands
Concept: Armored mobile force
History:
Steel Dragoons are a highly mobile chapter, relying heavily on their vehicles to deliver justice to the Emperor's enemies. Their chapter is reviled by the Adeptus Mechanicus due to their refusal to turn over a STC template fragment found during a crusade. The AM have stopped supplying Tech Marines to the Steel Dragoons, who have responded by committing what would be tech heresy by building and repairing their own vehicles and weapons. As a result of this schism the Steel Dragoons are equipped primarily in the MkIV Maximus armor. Due to their possession of the STC fragment their 1st Company is equipped entirely with Terminator Armor.
Wrath of Corax
Colors: Black/White
Banner: Cyberhawk
Motto: From the skies
Founding Chapter: Raven Guard
Concept: Boarding action specialists
History:
The Wrath of Corax is a fleet based Space Marine Chapter. The bulk of their tactical marines were wiped out in a trap by a treacherous Planetary Governor. With the bulk of their crowd forces lost upon landing, the Chapter evacuated their brothers and responded by cooking the planet's atmosphere off through orbital bombardment. The damage was done but the Wrath of Corax adjusted. While they can fight in planetary assaults the chapter has specialized in orbital engagements, boarding actions and ship to ship combat. Few enemy ships can survive when a boarding pod from the Wrath of Corax breaches.
Screaming Geckos
Colors: Bright Green/Silver
Banner: Angry looking Gecko head
Motto: There's no kill like overkill
Founding Chapter: Unknown
Concept: Heavy weapon obsessed maniacs
History:
The Screaming Geckos are borderline unstable, their solution to everything is the maximum amount of firepower. The chapter is notorious for its use of heavy weapons, no Space Marine chapter uses more suspensors than the Geckos. Their morale tends to be contagious, while other chapters question their sanity, nobody questions their effectiveness. It is not uncommon to see entire squads of Screaming Geckos armed with heavy weapons firing in to masses of Xenos, all of them laughing at the slaughter. Legend has it one of their chapter masters actually used a pistol gripped heavy bolter, but that is probably just tall tales.