I would purchase the clan stone and then double click it within my system to set up the siege (or do it from the journal if that's easier).
I would then have to set how we were paying for the wager (holding or money or a combo of both), if we were allowing people to join in on the siege, and who would get the property if the challenge is won.
Based upon these choices, the difficulty of winning is adjusted (as per the blog post). Once this siege "contract" is filled out, I can then go and drop my siege as usual.
When joining my side, a copy of the contract pops up and the joiners have to agree in order to join.