how do i convert rear drum brakes to disc brakes
You don’t. Maybe if the car was offered with them as an option you could expect to do this, but otherwise it is major surgery and, if you have to ask, you are probably way over your head trying to do it.
Why would you want to do such a thing anyways? To a ford focus nontheless. It’s easier for older cars where rear drums (and if it’s old enough front) drums were standard, and now it’s easier to find a “kit” for rear disk brakes. Most people just upgrade their current brakes with higher quality pads/shoes rotors or drums, even a bigger master cylinder or booster. I suppose you’re adding bigger tires/rims or plan on racing? Or towing??? Or this is a joke…