Running and listening to sound is so abstract. You can ask 10 audience members "How does it sound?" and get 10 different answers like: It sounds like an AM radio, or I can't understand the vocals, and even That's the best band I have ever seen.
Operating a sound console can be very abstract too. You can push up the volume slider for the guitar but what you really get depends on the settings of the Tone, Effects sends, the effects in the rack, the output EQs and the equipment in the Amp racks.
If you listen to music a lot then you know what you like and why. When you mix sound you should try to recreate the qualities that you hear in the music that you like. Make things clear and natural and use effects that compliment the music. Effects should sound naturally acoustic and don't over use them.
My favorite advice to give newbies in sound is to Listen carefully and don't use your eyes to adjust the controls. If you think the keyboard needs more highs and look at the control and it's already turned up, don't stop your self from adding the highs, and do not look at the controls when you adjust them. Listen carefully while you are turning the knobs and look at the musician while making the adjustments.
And lastly, when you close your eyes while you are mixing, you should be able to see the instruments. If everything is sounding clear and natural like it should be, then you should be able to easily visualize the instruments with your eyes closed.
You can't learn to run sound from a Manual or a Forum, you have to get out there and twist some knobs and listen to the results. I will be giving a Sound training class to the members of our theater and union later this month. I will have a system set up and we will twist and listen to every knob that I can reach.