I could write something so as to talk to a child but as you well know a child can not comprehend what the most of this subject is going to be. I think there is so much on a larger mixer that most people who are not used to them are overwhelmed.
The way to learn them is to sit down and study the manual and the parts that you should understand first is not the tiny detail stuff but the Block Diagrams. When I have a signal path question on my M50 or M520 I go directly to the block diagram as it gives me what I need to know not what Op Amps are used and that type of stuff.
On the mixer input modules you do not have 12 separate and different thing but really pretty much 12 duplicates of the first one. If you learn the path and process of this one input module then the others are for the most part the same so now you can understand the greater amount of the mixer by that method.
Next you then look into the Aux and submasters and Busses and even the stereo mix and Mono mix outs.
The audio going into this mixer is like going into 12 different funnels that have valves on them with a lot of outputs. In the 312 the outputs can be Buss (4 of them), L-R which is the Stereo buss, 2 Aux sends and a Effect send. When you open the valve to a funnel you are then assigning it to a buss- you have 4 of those to work with on this mixer or the item you are sending the signal to. The signal has to go somewhere. It just does not go out into space so the Effects for instance goes to the Effects master and then to the Effect out. What do you do with this out- whatever you want such as a plate reverb, spring reverb, Harmonizer or any piece of effect equipment you want to use. Mixers are all about versatility and how much you can do with them in the least amount of effort. Is there a standard way to hook them up? No, that is determined by the users and the other equipment he has- every install will be different.
If you understand the basics of the parts of the mixer, then walking up to any mixer no matter what size, you can understand the ideas contained on the modules. You may not know how it is wired in but they rarely spring a job on you where you don't have time to prepare for a session to learn what goes to what. There is a slate pad right above the fader on some mixers that are used to indicate what input the mixer gets it signals from. In your place you determine what to hook up to the input of the mixer. They often use a masking tape like tape and when you are done with a session rip it off and put a new piece down to wire in a new input. That is why there is a space right above the fader for you to label it.
Patch bays are nothing more than a lot of connection jacks brought out in front of you from another inaccessable place.
For instance, say your mixer is filled up and you have a DAT machine you sometimes want to hook into inputs 3,4 (L and R).
So instead of walking around or bending over the mixer to plug in The DAT cables and losing the RCA cables that were plugged in which is a real pain, you wire a set of cables from the 3,4 input to the patch bay. You bring the output of the tape deck and DAT machine into the match bay and when you want to change something you simply while still sitting down switch a couple of RCA jumpers around to accomplish the task. Many guys take all the tape outs into a patch bay so that they can add noise reduction with patch cables easier than going behind the mixer to do the same thing- many different scenes can be had- it is up to your design of all of this.
I can not give you the full course here but if you spend some time with a recording Engineer- he can in one meeting give you a lot of ideas as to how to best use your equipment. Then it is your job to go design the way you are going to hook all of this up. There can be a lot of trial and error and keep the amps down in case of unexpected feedback loops. It takes time and I was like you at one time but since learning I have built entire radio stations and help connect up others from other guys designs. Also many basement studios smaller than yours have been designed on the fly. None of those people were any smarter than you who used them.
If this sounds too complex then the only way for me to explain it would be by a meeting or training session.
