First step is to ensure your entire family is correctly registered in the Swiss electronic registers. (This is probably the case, so don't worry too much). Then you need to fill in an application form from the Mairie and send it in with the following documents (it sounds complicated, but actually very easy, just apply for each document by post and send proof you paid the fee):
1) Office de Poursuites certificates from the cantons where you lived
2) Police Certificate of no crimes, from Berne
3) Letters from the tax authorities of the cantons you lived in confirming your taxes are up to date.
4) Letters from the Cantons bureau d'etrangers where you have lived certifying or proving 12 years residence in Switzerland.
5) Fill in a CV form specifying every place you ever lived in, every school you attended, all your qualifications, every job you ever had, with dates. Do the same CV for your spouse and minor children, even if they are not applying for Swiss Nationality.
6) Letter from your employer.
Following acceptance by your commune that all the papers are in order, you will be invited to series of 4 interviews stretching over about a year. You can expect to be asked questions to determine your motivations, your future plans, your knowledge of the language, and you will be tested on things like the Swiss political system, geography, and current affairs.
You can expect questions like "Describe how Cantonal rules are made in this Canton", "How many people make up the executive", "Describe the Cantonal flag", "How is the Swiss parliament made up and who elects the members of the different chambers ", "What are the ways the Swiss citizens can influence the parliament", "What are the reasons you want to become Swiss?" , "What do you like about Switzerland?" "What don't you like?", "What is the largest enclosed lake inside Switzerland?" , "Name the main Swiss rivers", "What are the 3 initiatives to be voted on February 7th?", "What Canton is the most Eastern in Switzerland?", "When and where was Switzerland founded", "Name the most famous battles fought in this Canton and the outcome", "When and how did this Canton join the Confederation", "Which General was in Charge of the army during the Second World War?", "Name a famous Swiss teacher, poet, writer, sportsman,....etc", "What are the most famous sights in Switzerland?" "What local events do you attend?", "how do you participate in the community and mix with your neighbours?", "How much money, salary, pension, wealth... do you have?". (They probably already know what you put on your tax return).
You will be asked to nominate two Swiss Nationals who know you we'll, and can certify that you are a good guy.
Getting Swiss Nationality sounds gruelling, but it's a lot easier than passing a Chemistry exam at school. All you have to do is follow the Steps one by one, learn all about Switzerland, and fill in the forms correctly. I assume that you are pretty fluent in French? If not, better watch a lot of Swiss TV, until you are at ease.
Good luck!