One person (who will be the Clan Owner) needs to talk to the Scribe at the Clan Camp south of Falador to obtain a scroll that the other 4 players need to sign. Take the scroll back to the Scribe to let him know you have a full list. He'll ask you to enter your desired clan name, and that's all.
Best of luck with your clan.
If you have any questions about clans, I would recommend browsing this thread:
? Clan Help ¿
~ Sicariu
Leader of