Let's use a mic for this example:
Channel input gain down,
Channel fader down,
Connect mic to channel,
Sing into mic and raise input gain until loudest peaks are approx +6dB.
The input gain is now set for that channel.
When all channels are set, an 'ideal' mix would have all channel faders around 0dB
but of course you'll want some channels lower than that . . .to your own taste.
To set the gain structure through a system to the amplifiers, instead of a mic,
connect a constant tone -approx 1kHz (C two above middle C on a keyboard - flute without vibrato will do):
PFL input channel and raise input gain until you read 0dB.
Deselect channel PFL.
Raise channel fader until master LR reads 0dB.
Without knowing exactly what amps you have it is impossible to be specific but in a nutshell,
you now raise the master LR faders to 0dB and then, last of all, raise your amp levels to he volume required for the room.
If using a domestic amp, you may find the sound distorts with the master LR faders at 0dB.
If this happens, lower them until you get a good sound.
Now you know that the amps are getting whatever you see on the master meters.