well, I don't run a clan so I may be wrong. I would suggest you guys come up with a clan name and tag. All your friends should wear the tag while playing. Arrange time to play together and preferably use team speak to co-ordinate your play. It all depends on the purpose of the clan really, if you just want to start one for recreational purpose then it's really flexible vice-versa.
UKCS:TF2#3 - Medic
Don't shoot me, I am a Humanitarian ! =P