Use the mode switch found on the back of the amplification unit. There are three positions you can use ("Parallel Mono", "Stereo", and "Bridge Mono"). Use the "Parallel Mono" mode.
Locate the "Output" binding posts of "Channel 1" and "Channel 2". Use a 14-gauge jump wire to wrap around the red output binding post of channel two and then around the red output binding post of channel one.
Run the speaker cable from the red and black output posts of "Channel 1" to the inputs on your loudspeaker. Do not use the "Channel 2 Output" to your loudspeaker.
Use the "Channel 1 Input" to connect your source (guitar, mixer). Do not use "Channel 2 Input".