At least 6 weeks before the expected purchase completion date, you should make an application on JPA through the Self Service Application for FHTB. Full instructions can be found in the Applying for Pre-Approval for Forces Help to Buy user guide, which can be found at JPA user guides [this has to be accessed through defnet].
In exceptional circumstances, if you don't have permanent access to JPA, you should complete JPA Form E035, available via defnet at JPA forms and forward it to Defence Business Services, Military Personnel, FHTB Section, Centurion Building, Grange Road, Gosport, Hampshire, PO13 9XA.