Use a fork tool or ball joint separator and hammer blows to remove the lower ball joint from the steering knuckle. Special paellas and drives are available for ball joint separation.
Make sure that you have removed any other component (brake line, strut rod, steering linkage) that could be damaged when the control arm is lowered. Pull the spring and compressor out as a unit.
Install the compressor on a new spring, slip the spring into place and posit on the coil ends in the same location as the old spring. Reassemble the ball joint and other components. Then, unscrew the spring compressor while guiding the coil into it seats. Keep your fingers out from under the spring.
When replacing a rear coil spring, a spring compressor may not be needed the axle drops for enough to free the coil spring. Support the weight of axle on Jack stands or on a floor jack. Always reinstall isolators otherwise the new spring will squeak or rattle. Workshop manual of the under repair should be followed for exact procedure.