The best thing I've found is to just do it. After you read a book or two about the theory, I would get a wiring manual and start studying how your donor works. Having a compete harness and going through the manual wire by wire, labeling each connector, and understanding how the circuit works has always helped me. Seeing the right way to do it goes a long way (on that note I found some Lucas manuals online several months back...forget the site...I'd try to find them, read them thoroughly, and NEVER do anything they do EVER!
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There are only a few principles to really know - use relays to switch high current devices, never dump high current into the ECU unless told otherwise, coils charge when voltage is applied and spark when its taken away, and *most* automotive sensors work by varying electrical resistance (temperature sensors), providing a wave signal (crankshaft position sensors), or providing a varying voltage (O2 sensors).