Your Ford Focus has coil springs in the rear suspension. In order to replace them, you need a special spring compressor tool and adapters along with safety stands and a ratchet set. These steps apply to all model years of the Ford Focus except for the 2005 wagon.
Raise your Ford Focus and support it with safety stands. Take careful note of how the spring you're about to replace is oriented.
Place the spring compressor 204-167 with adapters 204-215 on the spring, compress the coil spring evenly and securely, then remove it (still compressed) from the vehicle.
Decompress the old spring slowly, evenly and carefully. If at any point you are unsure of how to do this, stop and consult your spring compressor documentation or a competent mechanic.
Compress the new spring slowly and carefully, then install it into your Ford Focus. Make sure that it's oriented as the old spring was. Be careful when decompressing the new spring, and continue checking to make sure that it is properly oriented throughout the decompression process.
Lower your Ford Focus to the ground and test drive it to check the new spring performance.
Always use safety stands; never work underneath a vehicle supported by a jack. Be very careful when compressing and decompressing automotive springs; they store a lot of energy. Never try to compress, install or remove automotive springs without the proper spring compression tool.