Search the web for Molex crimp right wrong and look through the pictures.
https://www.shearwater.com/wp-content/u ... _crimp.pdf is good too. Terminal crimps are one of the more human element issues found in harnesses that are hand made. You dont have to use molex terminals unless theyre on your board - good info either way.
Consider using tinned wire, this helps keep the connections & wire in good shape long term. Make sure its rated for higher temp and oil resistant.
If you're in the US Waytek wire is a decent resource for finding split loom, loom holders, etc. There are others too of course.
Relays and connections in the engine bay should be sealed unless they're boxed up or the vehicle will never see wet weather.
Shrink tubing can help identify wiring if you use all one color. If you can find multiconductor cable with striped and colored wires that will help keep you sane when troubleshooting or tracing the harness around.
Resist the urge to wire your fuel pump to the ignition switch or a on / off switch. Let the ECU run it. In a crash or fire you wont think to turn the pump off. You can do an on / momentary on fuel pump switch (on ecu control and momentary prime to help burp the fuel lines manually if necessary.)