Turn on the Marshall amplifier. Plug the amplifier's power cord into an electrical outlet. Look for two small switches on the amplifier. Most Marshall amps place the switches on the front side of the unit. Flick both of these switches upward. A red light flashes to signal the amp has been turned on.
Connect a guitar to the amplifier with an instrument cable. Insert a 1/4-inch instrument cable into the output jack on the guitar. Insert the other end of the cable into the "Audio In" jack on the amplifier. The audio input on Marshall amplifiers sits on the front side of the speaker. The amp produces a soft hum noise. Play a few notes on the guitar to check that the amplifier produces sound.
Flick the "Standby" switch downward to engage standby mode. The amplifier produces no sound. Standby mode prevents the guitarist from playing anything through the amp.
Flick the "Standby" switch upward to activate live mode. The hum noise returns, and the amp produces any sound played on the guitar.