Lots of good advice already - I would add a suggestion to start with twist ties for gathering your wires - easy to un-do if you move things around, or forget a wire or two, and let you figure out where you might want to put some wires into wire looms. Run every circuit to where it goes, but don't cut anything or terminate anything until you have them all roughly going where they will be including any turns or obstacles to go around (there will be some).
When you do terminate them, I am into overkill - I both crimp and solder, then cover the connection with clear adhesive-filled shrink tube. Makes very nice water-tight connections that you can see into - handy a few years down the road when/if something corrodes. I buy 4 ft. lengths of the stuff from DelCity.com. You can see when it seals because it will take on the color of the wire it's adhered to.
Run lots of grounds. You can do this - just do one circuit at a time. All together it looks mind boggling, but each circuit by itself, is simple.