Before you build: Choices and Comments
If you have a multimeter or voltmeter, it will come in handy for testing the power supply.
You have a LOT of choices with this project. I would not describe this as a beginners level, it is complicated by the use of surface mount components, having to stack the boards correctly, requiring a bipolar power supply, and mostly by the sheer number of options that have to be decided along the way. If you are not familiar with mixing boards, your choices get even more confusing. I'm not trying to put anyone off of this project, but I am trying to make it clear that this will take a little more than “slap the parts on the board and plug it in”.
There are a few references to “stack pins” in the documents. These boards are stacked on top of each other, and these pins connect them. You may choose from a few different options, described in more detail here.
The mixer can be set up s a “line mixer”, or a full “audio mixer”. These require different connections as you build the boards. They also use a different number of parts. I'll try to explain the difference. A “line mixer” has inputs for stereo pairs. Each pair (left and right) are mixed into the output pair (left and right). This is a simple way to combine many audio sources. Each pair responds to volume and balance controls (along with a few others). That's it, very simple. Each channel board can support two stereo pairs. 8 channel boards creates a 16 pair stereo mixer.
An “audio mixer” is set up a bit differently. Each input is a single channel (not stereo) and it can be routed between Left, Right, and two Effects Loops. It responds to Volume, Pan, FX1 and FX2 controls( along with a few others). This setup is more appropriate for a recording studio, where the stereo mix is being created. For a full mixer, we need one channel board for each input. 16 channels needs 16 channel boards. Four outputs from each channel are Left, Right, Effects 1, and Effects 2.
You need to figure out what you want. You may mix both the above in a single board if you like. The software can support each channel as line or audio mixer. This makes sense, as even with a full audio mixer, there is often an aux stereo input pair for joining boards together. You also have no need for effects sends on the effects return loop.
After the boards are built and powered up, you must configure them to match your needs. That is all explained here.
Choosing how you want to process your inputs is a huge topic. It is explained somewhat here.
If you can't make any sense of this, don't give up! Go to the forum and ask questions.