The Idea
Players are encouraged to be proactive organizers of play sessions. Think about what you or your character would like to explore. Would you like a mission that gives you insight into orkish culture? Would you like a mission that will reward you with blood pebble armor? Do you want to know more about a particular Passion? Have a particular idea for something that might be happening out in the world? Talk to a GM, and together you'll work out an appropriate adventure hook.
The Organisation
You will need to decide what Circle rating the mission should be (your own, plus or minus one), and what difficulty (easy, average, or hard). The GM will then go away and design your adventure. Meanwhile, you are responsible for scheduling the adventure, posting it on the Missions forum, and selecting the team of adepts who will join you.
The Terminology
Easy, Average, and Hard are used in the same way as in the Gamemaster's Guide: as a statistical reflection of a character's likelihood to succeed on a given roll. A Novice adept finds Difficulty Numbers of 1-2 Easy, 3-7 Average, and 8-12 Hard.
The Restrictions
Adepts must be within one Circle of the mission rating to be eligible. Therefore, a Circle 3 character may go on Circle 2, Circle 3, and Circle 4 missions, but is not eligible for Circle 1 missions.