How do I schedule a reminder email with Rules to users that have flagged an event?
Sounds like you know how to set up the Flag. Triggering a Rule with Flag is piece of cake and NodeOne has an create 15min tutorial covering all the details. They also have all the needed training on Rules if your fuzzy about Rules in general. Once your ready to set up your rule and you have the event to trigger it, you can set up the email action following a recipe similar to this other answer. In your case you have the future event. Lullabot has a tutorial specifically on the Rules Scheduler.