Clan Leagues. When and How?

I have heard that clan leagues will be introduced within time. When is this going to happen and how will this work? Does every clan have to select a certain number of members or are they all vs all, which is hard because not every clan has the same number of members. Are there schedules for challenges or do you have to make each challenge yourself. I really don't know how this will work. Can someone explain this?
Thanx,
Sander