This will show you how to make a 12v to USB (5v) adapter. The most obvious use of this is for 12v car adapters, but anywhere you have 12v you can use it! If you need 5v for anything other than USB, simply skip the steps about adding the USB ports ;)
You will need
More information on the fuse: you can use a higher rated one if you use a different transistor that
can take higher amperage. Because we put the fuse on the 12v side, (which can vary from 11.5-12.5 volts, we have to use a value 2.5x smaller than what we want on our USB side. So, if you want 1.5 amps for your USB ports, then you select a 0.6amp fuse, if you want 2.5 amps at 5v, you select a 1 amp fuse, if you want 3.75 amps, you select a 1.5 amp fuse, etc.). Also, if you really want to protect your circuit badly, just put one on both sides.
Of course, you can also use an existing 12v-5v converter in your car, be it light duty, medium, or heavy duty; or use these nice power adaptor that are meant to be integrated into other projects or this beefier waterproof, super high power alternative with a heatsink. They are nice because of their screw terminals.