It's pretty easy. The Home Office link tells you everything you need to know.
The first thing to do before applying is to book and do the Life in UK test. You can usually do it at Adult Education Colleges and LearnDirect places. It's a good idea to get the book first, as some of the questions are pure trivia.
The other thing to do is to collect all his days outside the UK since being granted leave to enter, along with documentation such as birth and marriage certificates, and photographs.
Next, fill in the application form, and either send it off in the post to the Home Office, or have the documents checked and returned at the registrar's office in your area that does these. If you use the document checking service, you get your passport back straightaway. If not, it goes into the ether until the end of the process.
Once the Home Offiec has the application, it takes them 2-4 weeks to adjudicated it. They then inform your County Council, who invite you to a citizenship ceremony.
Timings are about about 3 months start to finish, longer if there is a backlog with the Life in the UK test and document checking service.
Cost? It should be on the Home Office website, but ISTR around £600.