Inspect your arena sound system for visibly damaged parts and replace any that you find. Pay particular attention to cables, looking for exposed wire or broken rubber coating. A damaged cable degrades sound quality and is a fire hazard.
Ensure that you're using balanced cables to connect any instruments or audio inputs to the mixing board. A balanced cable has XLR-type plugs. An unbalanced cable has a quarter-inch plug. Unbalanced cables are far too noisy for use in an arena setting, so swap any that you find for balanced lines.
Set each channel fader on your mixing board to zero decibels. The numbers painted vertically next to each fader input on the mixer represent dBs. Individual faders control the volumes of individual sound sources, and the master fader controls the overall volume of the mix. Use these faders to control volumes during an event.
Connect any needed sound sources to the back of the mixing board, using one channel per device. For example, connect an MP3 player to channel one, a keyboard to channel two and the Master of Ceremonies' microphone to channel three.
Set the input level of each device so that its corresponding fader light reads below 0dB. Each device should have a similar input level feeding into the mixing board. For example, a keyboard, an MP3 player and an MC's microphone might all read at about -3dB.
Sound check your sound sources and make minor volume adjustments using the mixer's channel faders. Make sure you can hear each sound source clearly through your headphones. The idea is to give each source its own "space" in the mix, so that anyone can pick out each source individually while hearing them all.
Find the arena sound system's amplifier, usually mounted to a hardware rack near your mixing board. Ensure that all needed arena speakers are connected to the amplifier with speaker cables.
Slide your mixer's master fader all the way down and ensure that no sound flows through the output. Set the amplifier's volume to its maximum.
Turn the mixer's master fader up very slowly. Sound should now be pumping from the arena speakers. Tweak the master fader until you find the appropriate overall volume for your event.
Monitor sound levels carefully during your event. Control any feedback that occurs by turning down channel faders as necessary. Tweak the volumes of individual instruments throughout the event to find the perfect sound.