You simply make it, really.
Create a new thread, explain what the contest/collab is about, the submission rules or what do you expect from every submission, voting rules if people are going to vote, deadlines if you feel like having a deadline... and I pressume that's the basic skeleton of any contest/collab out there.
Do note that people aren't going to join a collab just because it's there - you're going to want to aim to make the collab fun or exciting for the participants. Making it a free-form "yeah just make a good level :)" is not really an incentive for joining nor a challenge to face.
It works for the OLDC because it's officially hosted by STJr and generally aimed to showcase what the community can do when they band together to make a high effort megawad.