I agree with the above posters - speak to a professional groomer before attempting it yourself. It is extremely easy to injure animals during grooming, especially cats since their skin is tissue paper-thin. Also any matting makes it nearly impossible to see the skin underneath. Sometimes skin can be caught up inside the mat, so if you try to trim it out you can easily cut it.
When I shave matted cats I only use a #10 blade. The larger blades (#7, 5, 4, etc.), and ESPECIALLY skip tooth blades are very dangerous to use on cats. The larger the blade, the more space between the teeth and it is very easy to have the skin get caught between them causing injuries.
Working at a vet and also talking to vet techs, there are way too many horror stories of home grooming going badly. This can be anything from skin irritations, to stiches, to surgery, to even death of the pet. This is an area that requires someone who has had training in this area. Ask your vet if they offer grooming, or can recommend someone who grooms cats. You can also check a website like findagroomer.com or ask if any of your friends can recommend a good groomer by you. Good luck!