The brake pads in your Ford Focus should need changing at 60,000 to 80,000 miles; if you do a lot of hard braking, you'll need to change them sooner. The 2005 Focus uses Teves brake calipers, which hold the pads within the caliper itself rather than a mounting bracket on the disc. Even with actual knowledge of the procedure, changing brake pads is an important and difficult job. Consult with your mechanic first.
Raise the car's front end and support it on jack stands or axle stands; make sure the parking brake is applied. Remove both front wheels.
Pry out the outer retaining clip on the caliper with a flat screwdriver while holding the clip with pliers. Release the caliper hose from the bracket on the strut, but don't disconnect the hose from the caliper,
Pry off the plastic covers from the ends of the caliper guide pin bolts and remove the guide pins with a 7 millimetre Allen wrench. Lift off the caliper and support it on an axle stand where it won't strain the hose.
Remove the inner pad from inside the caliper; it's held by a retaining spring inside the piston and needs pulling out. Slide the outer pad out of the caliper with its securing clip.
Clean the caliper with brake cleaner, using a dripping pan to catch the residue.
Compress the caliper's piston into the caliper bore with a C-clamp. Keep an eye on the fluid level in the engine's brake master cylinder reservoir as you do this, and siphon out fluid with an unused syringe if needed to avoid overflow.
Install the new brake pads in the caliper, sliding the outer pad in place with its securing clip and inserting the inner pad into the piston bore via the retaining spring.
Attach the caliper back in place on the disc and apply the guide bolts, tightening them to 21 foot-pounds.
Attach the front wheels and lower the car once you have changed the brakes on both wheels.
Press the brake pedal multiple times until the brakes feel firm, properly seating the brakes.
It can take 100 miles or so until the brakes are fully seated. Avoid hard braking during that time.