A critical feature of your new wiring harness is missing from this article ... the wire to use (and what not to use).
You should ideally use SAE spec wire, which is exactly what the OEMs use ... and there is a trick to this as well. SAE grade wire is not AWG sized. Because weight is always a consideration in an auto, SAE specs wire gauge to be slightly lighter than AWG gauge (we're talking about the copper content, not the overall diameter, although that is a bit smaller as well than equivalent number AWG wire). So be sure when you use the usual tables for current carrying capacity (most serious car stereo websites will have a chart) that your SAE wire is adequate for the length and current necessary (current from your calculations in the article). In some cases you might want to increase the SAE Gauge one step versus AWG.
What not to use? There are many appropriate alternatives to SAE grade wire, although they might be more expensive. The more common substitute would be teflon-dielectric* silver-clad copper aircraft / aerospace wire **, which race teams often use. You can buy it in bulk spools, but as in the ac / ao industry they are only allowed one splice per run of wire, (think of a large passenger aircraft and how long a run might be) so they will always have remnant spools and it is found on the surplus market for good prices. But there might be others you could consider. Why that particular wire? It's used in the engine compartment and any other high heat area as it can withstand very high temperatures without melting the dielectric and more importantly, where SAE wire would melt the dielectric (PVC).
Another wire you might encounter in automotive stores is Copper Clad Aluminum (CCA) wire. It is cheap and light, and many China-sourced large gauge cabling is CCA. Look for those letters and avoid at all costs. It "looks like" copper so be wary. Aluminum is brittle and breaks easily and only carries about one quarter the current at equivalent AWG as copper. Very common on cheap jump start cable sets, which you might have bought inadvertently. The rule of thumb is subtract 4 AWG gauge sizes to be equivalent current capacity as copper. So your "4 Gauge" Wall-Mart jump start cables are really equivalent to 8 Gauge copper. If you inadvertently used it for battery / starter / etc circuits, you have made a serious compromise in your electrical system capacity. *** Avoid.
Do not, under any circumstances, use "Primary Automotive Wire". This grade wire is rated only for wiring trailer wiring and connectors attached to the outside of the vehicle frame. It is a very low grade of wire and won't last, and is not appropriate for underhood or interior wiring. You will be setting yourself up for the same failure you're trying to eliminate by going through the re-wiring process in the first place. If it's not part of your trailer wiring, don't use it. Full stop.
The problem with PAW is it's often the only wire you can find at any automotive store. So do your wire order online from a site that specializes in automotive wiring applications. They will often have spool kits where you might choose 10 spools of different colour and the same SAE gauge, which makes a colour coded harness easier.
You can buy makers in little books that are numbered or lettered, and create markers for same-coloured wire, or just do so for ease of maintenance later. There are also ones that just have sequential numbers to mark your wiring diagram and the wire. There will be multiple examples of each number or letter so you can mark your run at specific intervals (every three feet, for example) or at the terminating ends, or both. Electronic supply shops like DigiKey or Mouser will have them. You can also mark single-colour wire with a line along the run with a sharpie to make (for example) a red with black stripe wire. This saves you the trouble of getting too many different coloured spools of wire.
RE: Solder vs Crimp
I actually often use both solder and crimping. "Tinning" a bare wire with solder has certain advantages prior to crimping. In some cases you can solder after crimping as well if you use bare crimp connectors and cover with heat shrink. The reason OEMs don't solder is because it's not conducive to the manufacturing workflow, and also because a soldered wire is stiff and there is a chance a wire will break right at the soldered / bare point. If you are careful and avoid bends at that point (think about maintenance as well if you disconnect a connector) it's fine and may be superior to a crimp alone.
A nice straight section beyond your solder point in your layout can be used to mitigate the problem, as well as judicious use of shrink tubing as a strain relief (you can use multiple layers of shrink to stiffen a strain relief). You can buy "kits" with double-layer adhesive lined shrink tubing which is excellent for outdoor applications as it's air and water tight, while it's cheaper as single layer without adhesive 4 foot sections or spools. You can add adhesive to plain shrink by using a small blob of silicone sealant before you shrink, but use 3M electronics grade silicone if you do, as ordinary silicone sealant can be corrosive to electrical connections. Up to you. A bad crimp is worse than a soldered connection, but a good crimp is very good electrically. Do what you feel is appropriate at each specific connection.
Someone mentioned lead solder (tin-lead) as the ROHS directive in the EU has pretty much eliminated lead solder in many applications and substituted some form of tin-silver solder. All you need to know is tin-lead is much easier to work with, and for automotive applications ROHS does not apply, even in the EU. Tin-Lead is also mandated for aerospace and military applications under ROHS. Kester 44 is an excellent brand / formula of tin-lead, but there are other brands that are fine. Choose "eutectic" formula which is 63/37 mix, rather than 60/40, as it transitions from solid to liquid better which makes working with it easier. Buy a spool if you intend to solder anything.
To reduce the chance of water ingress at certain points, you can layout a cable with a loop at low points on the run. Water will then drip from the bottom of the loop, instead of into your wiring or connectors. Do not bend too tightly, a nice no-strain loop is preferred. A couple of inches in diameter usually is good. Doing so also helps avoid strain by too tight a run, so if your layout has extra length (and it should) add a loop so your layout is clean but you can still disconnect something and work on the vehicle.
The reader who suggested a sheet of plywood for layout made an excellent point. I sometimes use shelving planks from the usual hardware stores instead, handy for long layouts that might extend beyond 8 feet. Also up to you. Don't be afraid to use your sharpie to mark your layout before you actually build your harness on it.
You can buy inexpensive stiff PVC-coated wire for tying plants to stakes (like your garden tomatoes) from hardware stores or (my favorite) your local Dollar Store. You can use it to lay out a proposed wire run on the vehicle, and then transfer that to your harness layout on the bench. Makes it easy to deal with bends and routing without trying to measure it with a tape. Another cheap tool is a sewing tape, they are fiberglass and last forever, and 60" long (usually). Maybe $1~$2. Makes it easy to measure non-straight layouts.
* Dielecric: The technical name (it describes the electrical properties) of what we mortals call "insulation" on wire.
** The silver cladding on each wire is not there for electrical properties, although silver is slightly better current capacity than copper. It's there for corrosion resistance; bare copper is subject to corrosion mostly due to atmospheric conditions (contaminants and moisture in air). You know ... that green corrosion on bare copper. Low Grade PVC wire might not be gas tight either ( cough - primary wire - cough) so corrosion can creep up well into the covered length, which embrittles the wire and leads to "invisible" breaks inside the coated portion of the wire.
*** Reasonably priced large gauge wire can be found at welding supply stores. Be sure to carefully lay it out if you use it, as the rubber dielectric is not particularly resistant to cuts and cracks. You can coat it with heat-shrink tubing for added durability. Avoid sharp bends and always use grommets and appropriate sized wire clamps that bolt to mounting points, not wire ties, to route it. You can solder & crimp large gauge wire, but will have to use a heat gun to pre-heat that much copper before soldering.