Bizarrely you do not have to do anything other than write in, with your name and address, and specify the information you are requesting. The recipient then had to don their clairvoyant hat and guess whether the request falls under FOIA or not (because if it does they have to record and process it in a different way to general correspondence). It may well be a lot quicker to say in your letter something like 'I believe I can request this information under the FOIA'.