For the underhood:
Step One: Buy a Factory-style harness and diagram. I bought the one on ebay with a picture of my Dart on the front, and I keep it in the car, with all my modifications drawn in.
Step Two: Read up on the ammeter and bulkhead bypass.
Two A: Disconnect battery.
Step Three: Unless it's cherry and you can inspect it all, consider replacing the dash harness too, which you'll wanna do after you do:
Step Four: Unplug one harness at the bulkhead connector only. Inspect bulkhead connector for anything green or melty. Install new harness at the bulkhead connector only. (my connectors were such that only one would fit one location)
Step Five: Work from the bulk head connector area out. If the original harness is in a retainer, undo it, and lay your new one in and re-affix. If it's connected to something, disconnect old and reconnect new. Keep going until your old harness is out and the new is in, one component at a time.
Working so in such a manner makes this a monkey-rate job that you'll probably enjoy. I certainly enjoy laying the harness in nice n' neat. Hardest part is keeping everything untangled. Pack your new bulkheads with dieelectric grease, consider doing the above bypasses, keep fire extinguisher in the trunk because duh. The hardest thing about doing the dash is working through the instrument hole and glove box hole to reach the junk in the middle but it can be done.