Set up three microphone stands. Adjust two of them so that the collar is positioned an inch away from the speaker grille, directly in front of the center of each speaker. Shine a flashlight into the amp to identify the center of the speaker. Set up the third stand approximately 12 inches in front of the amplifier, centered between the other two stands. This position will capture the sound as it enters the room space.
Connect an XLR cable to each microphone. Push the female end of the cable into the three-pin socket on the bottom of the microphone. Slot a microphone into the collar of each microphone stand.
Connect the male end of both XLR cables to the XLR input jacks on your audio interface or mixer. The audio interface and mixer serve the same purpose, to convert the audio signal into data for recording on computer.
Open your preferred audio production software, for example Logic, Reason or Cubase. Double-click the icon on the desktop to open the program.
Assign each microphone to a different channel. Click "File" and select "New Audio Track." Type "3" into the box to open three tracks. Name the channels so that you know which microphone is assigned to which channel. For example, "Close Left," "Close Right" and "Room." Turn the "Pan" dial on the "Close Left" channel to the extreme left. Turn the "Pan" dial on "Close Right" to the extreme right. Set the "Pan" dial on "Room" to the center, this configuration creates a stereo image.
Test the levels. Plug your guitar into the amp and set the dials to your preferred settings. Play as loud as you intend to when you record. Monitor the meter on your audio interface or mixer. If the red LED illuminates, turn down the "Gain" dial on the interface or mixer.
Turn off any fans, air conditioners and any other noise-making objects so that they do not spill onto the recording.
Click the "Record Enable" icon on the channel strip for each microphone. This is typically a red "R" icon to the right of the channel name. Click the "Record" button at the bottom of the screen and play your guitar.