We keep the rosters enlisted in the Ops and Challenges for BO2 & MW3 in case anyone is interested in participating, but we mostly earn points by coincidental gameplay. We might happen to be playing TDM when there is a TDM challenge and earn points.
If it is listed as a "Challenge" then it is a 24 hour event and rewards are broken up into three levels (Bronze, Silver & Gold). For example, the current "Total Wins" challenge ends at noon est today. All the wins for each member of the roster are added together, if the total is more than 25 the clan earns 100 clan xp, if the total is more than 50 the clan earns an additional 100 xp, if the total is more than 75 the clan earns an additional 100 clan xp for a total of 300.
On the weekends there are "Operations" that last for only 3 hours and the top 6 scores for the clan are added together and compared with all other participating clans to see what level of clan xp is earned.