Start by determining whether you fit the bill and meet the criteria for applying for an FFL. Applicants must be 21 years of age or older, have appropriate safety devices and secure gun storage, have no record of prior violations of the Gun Control Act, not be under any prohibitions from possessing, shipping, transporting, or handling ammunition and/or firearms, and the premises which will be used for the sale of guns and ammunition need to comply with state and local license requirements.
Once you have ascertained that you meet all the requirements, proceed to visit the ATF site and find the appropriate form to fill. There are several firearms license types available, but ATF's Form 7 (5310.12) is the one that applies to most people and businesses. Download Form 7, the FFL application, and the Certification of Compliance – these are all available in PDF format – and obtain the FD-258 (these are Fingerprint Identification Cards).
After you have obtained the entire application packet, fill out all the forms completely, clearly, and honestly (the ATF conducts background checks on anyone applying for a license, so do not provide incorrect information). You will also need to obtain 2x2 inch passport size photographs for every Responsible Person (RP) you have listed on the application.
Once the application has been completed, put everything into a packet, and mail this to the ATF processing address mentioned on the application form, along with the required application processing fees. The application will be handled by a field office assigned specially for the task, and will take around 60 days to process. During this time, the ATF officials will conduct personal interviews, background checks, and field inspections – if you pass these, you will be granted a Federal Firearms License.