If you search using the term ilr you will find a few threads with lots of information on this subject.
Basically you need to dounload the latest set(m) application form which you need to fill in, you can either send that and your evidence in the post or book an appoiment to go to a peo and hand in the form and evidence in person. It costs more at 950 pounds as opposed to 750 pounds by mail.
You will need 20 pieces of evidence (letters and bills) proving that you have both been leaving at the same address over the last two years or so from at least 5 different sources.
I would advise you start researching and compiling everything as soon as possible.
