Place a large-diaphragm condenser microphone in front of the left speaker, approximately four inches away from the center of the speaker cone. If the speaker has more than one port, center the microphone between each of them.
Repeat the process with a second large-diaphragm condenser microphone on the right speaker in the exact same place as the left speaker. The microphones must be the same distance from the source to capture the same amount of sound from both speakers.
Connect the female side of the first XLR cable to the microphone in front of the left speaker. Connect the other end of the cable to the first input on the stereo mixer. Turn on the phantom power on the mixer channel if it is required by the microphone. Phantom power is a 48v boost in signal some microphones require to capture an audible amount of sound.
Connect the female side of the second XLR cable to the microphone in front of the right speaker. Connect the other end of the cable to the second input on the stereo mixer. Turn the phantom power on if required.
Connect the stereo output of your mixer to the input of your stereo recording device with the cable. The type of cable needed depends on the particular equipment you're working with, but most consumer grade mixers and recorders will accept stereo RCA connections. Put the stereo recording device in standby or monitor mode.
Play the audio that you wish to record and increase the volume on the first and second channels of the mixer the same amount until the stereo recording device shows a level close to 0dB at the loudest point of the recording. Anything over 0dB is considered distortion, and is generally unpleasant to listen to. This step may take several minutes.
Push record on the stereo device and then start the source material you wish to record. Stop the recording on the stereo device once you have completed the recording or wish to start a new track.