The Home Office website offers a comprehensive guide to becoming a British national, as well as the necessary application forms.
All application forms are in PDF format. Print off the form and return it by post (details of where to send the form are on the page you print from).
To access the form you will need a copy of Adobe Reader. Information on how to download this for free is available on the right panel of this page.
To view PDF files you need Adobe Reader. The program is freely available if you do not already have it.