How do you convert 12 volts to 5 volts DC?

Voltage divider is ok if your load is only few milliamps. As more load you have the more power (or heat) the series resistor has to dissipate and of course there will be voltage drop on the resistor. This means that it will be difficult to regulate the output.

2 options: 1. Use linear voltage regulator such as LM340T-5.0 or 7805. The most common version is the 1 amp version but there is a 3 amp and a 0.5 amp version. Because this is linear, heat must be dissipated. You need large heatsink if you have high load. 2. Use switch mode regulator such as LM2677. Because it is switch-mode the efficiency is close to 100, thus little heat si dissipated even at full load. Most of the time no heatsink is required and the whole circuit will fit inside the cigar lighter plug.