It's just like connecting the dots
I usually go through 3 or 4 times as many wire ties as will end up on the harness when I'm done. I start in the center, like you are doing, then pick my routes and then add wires as the systems get added. Keeping them bundled with wire ties is easy, just cut off the previous ones when you add new ones to the bigger bundle. I usually add a wire or 2 to the bundle that goes to each end of the car that is unconnected at each end, just in case I want to add something later.
Try to not use those stick on plastic pads to secure the wire bundles to fiberglass. They will inevitably fall off unless you glue them with some epoxy.
Tea's makes a nice seat, don't you think?