Automotive wiring has been standardised over the decades, and most cars will have "colour-coded" wiring for lighting, radio, ignition, and secondary systems. This colour coding makes it easier for the layman to quickly repair most automobile wiring issues. You can learn to understand the wiring diagrams by following some basic principles.
Locate the source of the electrical problem, such as a damaged relay. Find the location of the issue shown in the diagram. The repair manual will provide troubleshooting steps to determine which electrical component is damaged or malfunctioning.
After locating the faulty part on the diagram, usually by labels or shape, the colours of wire on the page should exactly match the wire colours in the vehicle. This will assist in tracing the wire back to it's power source, or on to the next electrical circuit. Symbols on the diagram could vary, but typically follow electrician standards.
In rare cases, the diagram could be in black and white, or printed without colours. This is when the "path", or lines on the schematic become important. Sometimes, there may even be very small text next to each line, designating what colour it is. This can appear as "DRK GRN" for dark green, or "YEL/BLK" for a yellow wire with a thin black stripe.
Use a circuit tester to ensure polarity.
Disconnect the vehicle's battery when any wire repairs are to be made.