You need to have a level 85+ character first. From any 85 character, you can go to EQ button -> Quests -> Overseer to open the Overseer window (note: in Options, you can assign a key command to open the window). Go through the tutorial quests until the end ... and bam, you can start Overseer-ing. Overseers works by account by server so any 85+ character on your account can access Overseer window. When you claim a reward from an Overseer quest completion, that reward is bound to that character (although the reward items including coin are generally heirloom and transferable).
Click a quest, click an agent, slot, click an agent that shows up in bottom, and keep adding agents until you have all the agent slots filled. Then start quest.
Decent way to keep some progress going through the long levels.. I use it to get nearly 10% exp a day by doing 2 x 12 hour quests every day, currently lvl 109.
