It's recommended that you send a written request for a 504 eligibility determination to both the 504 Coordinator (usually someone in the special ed office) and the 504 compliance officer (usually the principal). Send it by certified mail. Seriously. Then, they'll contact you to set up an initial meeting. Bring all the evidence that your child qualifies for a 504 plan (that they meet the critera for a disability as it's defined in Section 504). Maybe even bring a signed letter from a doctor and test results.
Usually the first meeting is just an elibibility determination. After all the parties (could include school psychologist, nurse, teachers, etc) agree that the child qualifies, then they set up a meeting to discuss accommodations. It might take several meetings to get all the accommodations worked out. You will definitely be a part of these meetings, and should take the lead in suggesting accommodations.
After the plan is worked out and signed by all the teachers, the 504 compliance officer (principal) is responsible for making sure the accommodations are being carried out.
Contact your local PTI center for support and printed resources. http://www.yellowpagesforkids.com/help/ptis.htm
They may also be able to direct you to a parent advocate who can help you through the 504 process. But just be aware that some parent advocates are employed by the school district, so they may not really be all that helpful....