Making cards and playing magic, apparently. It's what we're going to be doing today. This is going to be fun. Let's go through what you're doing step by step, then make some rules.
Step 1: Choose your faction, or make a new one. A faction can be a tribe (Humans, Vampires, Goblins) a broader color combination (Jeskai, Rakdos, Grixis) or something a little different (Elder Dragons, Phyrexian, S.N.E.A.K.). Along with choosing your faction, make some lore for them if their new, and make a reason for them fighting if they aren't. If you have a question, just ask. Oh, and 1 person per faction.
Step 2: Create a Card for your general, or leader of your faction/army. Legendary, around 10 CMC, should have mechanics setting the rest of the faction and buffing your other dudes. Your Generals will probably end up fighting, so just buffing other units is probably not enough.
Step 2: Create cards for your Officers/High Ranking Soldiers. 3 legendary cards, around 6 CMC. Officers are more buffing other dudes, high ranking soldiers are more fighty dudes.
Step 3: Create cards for your soldiers. The rank and file, 2-3 cards as well, non legendary. If 2 cards, around 4 CMC, if 3, 3 CMC. If you want to just make these Vanilla creatures with pushed stats, that's fine.
Step 4: Create support. Just make 1-2 sorceries. Same CMC rules as soldier, just 1 less card.
Step 4: WAR. Untap.in, put all these cards in custom. You will be fighting in Untap, attacking creatures is allowed, and you have 10 lands. Your deck consists of all soldiers already on the battlefield, the rest in your hand. 4 copies of each soldier.
1. No changing the steps above, unless your army relies on it. If so, ask me.
2. Make your cards balanced.
I'll organise any battles, and I'll be making my own faction. For last. If you want to make new cards inbetween battles to represent change, say you want to make a new leader or promote a officer to leader after your leader died, go ahead. That's cool. Rewards will be decided based on how many people participate. I'll probably end up favoriting all of the cards made for this challenge, though. Good luck, and have fun!