I am sure you heard about it, but we have a room only for people who are interested in keeping the site clean!
That is the room that was used a year ago for the cleanup efforts and works like charm. There you are discuss posts that should be voted for any reason, edited, answers that should be up voted for closure and flags.
Feel free to discuss there flagging with moderators (you can ping us) and with other users present on the room.
The reason I recommend it is that flagging is not always straight forward, you cant be a bot over it. Sometimes its not clear as water, experience and good judgement will teach you about it, and that comes with practice.
Sure, we can write some guide lines, but that wont go far from the short paragraphs present on flags. Dont be scared of flagging, even if they return rejected -heck, every moderator has some returned flags of their own-. Its a learning process I would say, and I am not really sure a room to teach how to do it will help that much.