If they aren't installed permanently, place the mixer and the power amplifier in the desired locations in the sound booth area. Place the DBX compressor nearby (or install the compressor in its permanent location).
Plug the mixer, the power amplifier, and the DBX compressor into the power outlet. Connect the mixer to the power amplifier and the compressor to the mixer using the patch cords.
Plug in the studio monitors to the amplifier (if not already permanently installed and plugged in). Turn on the mixer and the compressor, but not the amplifier.
Set all volume pots (controls) on the mixer and the DBX compressor (if any) to 0. Connect the source signal to the mixer in the desired input channel using patch cords (recorded media), microphone cable, or instrument cable.
Set the amplifier and compressor volume pots to a low level (1, 2, or 3 as needed) and turn the amplifier on. Cue the source signal (start the recorded media, test the microphone, or play the instrument).
Slowly raise the volume on the mixer until the source signal can be heard sufficiently without feedback.
Adjust the settings of controls on the front panel of the DBX compressor one at a time. Experiment with each control setting as the source signal continues to play in order to become familiarized with its functions and to maximize the potential of the compressor.