Probably the best place to get started with a 504 is over at the Food Allergy Support Message Boards here.
They have done an absolutely amazing job compiling and organizing tons of great information on 504 plans, and how many people deal with their schools and keeping their children safe.
they also have a great list of other links to look at to get even more info on 504 plans. Some of the ones I really like are
This is the government's Office of Civil Rights information on just what a hidden disability is, who qualifies, and what the schools need to do to help keep the children safe.
This is from the Food Allergy Initiative which has wonderful information on a wide variety of topics relating to food allergies. I really like this article because it goes over what sorts of questions you should ask yourself when trying to come up with your own child's 504 plan. There are even formatting examples of how you should do some of the wording in the document that you can use when writing up your own plan.
This is a great site that gives you a simple straight forward overview of what a 504 plan is and who it applies to. It explains the difference between a 504 and IDEA (Individuals with Disabilities Education Act), how they relate to each other, and which plan applies to what disability.
So there you go. Don't get too glued to the page that you stay up all night.