Instructions
Gather a group of friends and agree on a clan name. This doesn't actually have to be a group, but there should be at least one other person to create a clan.
Create a plan on how the clan will be run. There are a variety of tasks that need to be decided, depending on how involved the clan will be, including ranks of officers, members responsibilities and etiquette, play times and recruitment requirements.
Agree on clan colors. Since there is no formal recognition of clans in "Halo," often such groups use coordinated armor color to symbolize unity.
Create a group on the official "Halo" forums to advertise the clan. Groups are made by logging into the official website using your gamertag or email and password, then clicking on the "Groups" link and selecting "Create New Group." Enter the clan's name, motto and description and click "Create Group." The forum account must be registered at least two weeks before creating a group. Add members to the group as needed. The group can also create a standard signature for all member postings on the official boards.
Build a clan website. Often clans will use their own website as an off-game meeting place to discuss their in-game exploits, post pictures, plan sessions and generally converse while outside of the game.
Start a new Xbox Live account under the clan's name. Add the clan members as friends to the account. This dummy account makes it easy to see who is online at anytime. Each player can also use the same Xbox Live logo to signify solidarity with each other.
Recruit additional members. Once the clan is established and the name, colors and membership requirements are all settled, go onto the forums or in the game and begin advertising your group. Players who wish to join should be given a tryout to make sure they are up to the clan's standards and if they're someone with whom the rest of clan will enjoy playing.