Are you asking for help for a DIY project from a different supplier? This particular forum is supposed to be for help with projects from the MBP store.
I suggest you look at the building instructions for some of the "noob" boards in the MBP store, since they have good diagrams showing you how projects are connected to the input, output, and power jacks. Since you are building a "splitter/mixer" you can think of any "SEND" points as a type of output jack, and any "RETURN" points as a type of input jack for your wiring.
I hope that is not only helpful, but gets you acquainted with some of the projects available from this site (no affiliation with me, except that I have built a number of them and have them on my pedal board).
That should be enough to get you up and running. If your project is not working after you get it together, you may need to ask for assistance from the seller's site.