You just send a letter to the organization that you believe would have custody of the information you seek. Make sure the letter states that the request is filed under the FOIA.
Before sending the letter, you should check the website of the organization. Many government offices have a section of their website detailing exactly how to file a FOIA request for that office. That section will also usually give you an indication of fees that will be charged for photocopying and researching the information.