The rear brakes on a Ford Focus are called "drum brakes," which differ from the front wheels (called "disc brakes"). The brake pad, called a "shoe," sits inside of a "drum." When you press on the brake pedal, hydraulic pressure pushes the shoe against the inside of the drum, helping you to slow down. Eventually, the shoe will get worn down through constant friction caused by braking. When the brake shoe material is less than 1/8 inch thick, you will need to replace the shoes.
Break the lug nuts loose on the rear wheels. You are simply trying to make it easier to loosen the nuts when the car is off the ground (and the wheels can spin freely), so you do not want to actually loosen the lug nuts to the point where the vehicle may become unstable or the wheels are in danger of coming off under the weight of the car.
Put the car in 1st gear, if your Focus is a 5-speed (it should be in park if it is an automatic). Put the car's emergency brake on.
Jack up the car using the rear jack point on the Focus. it should be in the middle of the car under the boot area.
Place two jack stands, one on each side, under the pinch welds in the rear of the vehicle. They should be located on the side of the car in the rear of the vehicle. Alternatively, you can place the jack stands underneath the frame of the car. Lower the car onto the jack stands and check the car to make sure it is stable.
Continue to loosen the lug nuts and remove the wheels/tires from the car.
Remove the four 13mm bolts that hold the spindle on. You can then take the drum off. Depending on how old the car is, you may need to hit the drum in order for it to come off. To do this properly, take a standard metal hammer, and hit the side of the drum all the way around. You are trying to knock the rust/corrosion off of the drum (which is keeping the drum in place) so that it is free to come off.
Using the brake spring tool, carefully remove the return spring for each of the brake shoes (highlighted in yellow).
Hold the back of the retainer pin and place the brake shoe removal tool over the retainer clip (highlighted in red). Press "in" and turn the tool counterclockwise. This will remove the spring and the retainer. The brake shoes (highlighted in green) should come right off at this point.
Place the new shoes on the brake drum and reverse the process you used to remove them in step 8 and reattach the return springs.
Put the outer part of the drum back on and replace the four bolts and spindle you removed in step 6.
Put the wheel/tire back on, and hand tighten the lug nuts. Using the lug nut wrench begin tightening the lug nuts so that the wheel/tire sit firm against the brake drum assembly, but do not tighten them all the way. Be sure that you are tightening the lug nuts in a criss-cross pattern. For example, tighten one nut, then tighten the nut opposite of it.
Lower the car back to the ground. Torque the lug nuts to 100ft lb using the same criss cross pattern you used when tightening the nuts initially. Before you drive away, check the brakes by pumping them a few times to ensure that normal brake pressure is being applied by the system.
Do not jack up on the boot or the gas tank, as you could rupture the tank or put a hole in your car. Do not place jack stands under the body of the car, under the gas tank, or under the boot area. The only secure areas to put the jack stands are under the actual frame or the pinch welds.