Insert a compressor into your guitar’s signal chain. If you are playing the guitar through an amp, this can be done with a guitar compressor pedal. If you are capturing the guitar with a microphone for live or recorded sound, you can use an analog or software compressor to accomplish this task.
Set the threshold of the compressor so that it is just above the volume of the harmonics. When the guitar level goes above the threshold, the compressor will begin attenuating the signal. This will allow the harmonics to go uncompressed, while compressing all of the other guitar parts that are louder than the harmonics.
Select a compression ratio to determine how heavily you wish to compress the guitar signal. A low ratio of 3:1 will allow the guitar’s volume to increase 1 decibel for every 3 decibels it goes above the threshold. A high ratio of 10: 1 will allow the guitar’s volume to increase 1 decibel for every 10 decibels it goes above the threshold.
Adjust the attack control of the compressor to determine how quickly the compressor responds to the guitar signal when the volume goes above the threshold. A fast attack time will allow the compressor to quickly latch on to the guitar signal and begin compressing it. A slow attack time will let some of the guitar signal through before it begins to compress it.
Adjust the release control of the compressor to determine how quickly the compressor responds to the guitar signal when the volume goes below the threshold. A fast release time will allow the compressor to quickly let go of the guitar signal. A slow release time will allow the compressor to hold onto the guitar signal a bit longer before letting go.
Adjust the output gain to make up for the volume lost through compressing the louder guitar parts. If the compressor is attenuating the guitar 10 decibels at its loudest section, increase the output gain 10 decibels. This will bring up the volume of the uncompressed harmonics, while keeping the peak level of the other guitar parts the same.