Building the matrix takes time. Like, a lot of time. I methodically moved through and used Nathan's very helpful diagram
to create the matrix.
Heat shrink every junction of wire to LED lead, as you don't want ANY MOISTURE creeping into your circuit. I heat shrunked (that's a word, right?) every LED, and then wrapped it in electrical tape.
It was helpful to use flux to get the wire junctions to solder properly together. I used a junky paintbrush to coat the leads as I was soldering. I also made hundreds of Jumpwires using my Jumpwire Jig
Did I mention this part takes time? It took me like four nights of mellow music, and a few beers, to do this part.