Adjust the "Input Gain" of the compressor to set the desired overall volume of the rap beat.
Adjust the "Threshold" control on the compressor so that the compressor turns on during the passages of the rap beat that are too loud. The "Threshold" determines at what level the compressor turns on and off. When the audio level passes the threshold, the compressor will turn on and compress these louder parts of the rap beat.
Set the "Compression Ratio" in order to determine how much the rap beat will be compressed when its level passes the threshold. For instance, a compression ratio of 4:1 will allow an increase of 1 decibel in volume for every 4 decibels that the signal goes over the threshold. Choose a lower compression ratio of 2:1 for more transparent compression. Choose a higher compression ratio of 8:1 for more aggressive compression.
Adjust the "Attack" and "Release" controls to set how fast the compressor turns on and off when the level of the rap beat has passed the threshold. A short attack time will cause the compressor to turn on immediately after the signal goes above the threshold. A short release time will cause the compressor to turn off immediately after the signal drops below the threshold.
Adjust the "Output Gain" of the compressor to make up for volume lost through compression.