Creating a powered USB hub, trying to drop my voltage from 12v to 5v (AC/DC converter I have outputs 12v)
The absolute easiest way to produce 5V out when you have 12V in is with a 3-terminal voltage regulator IC, LM7805 or equivalent. Search for data sheet or articles on that chip for suggested schematics (which may include one or two capacitors), depending on what's already on the USB hub PCB.  But then again, you might look a little closer and find that the USB board is intended for a higher voltage AC adapter, and already has a comparable regulator on it.

A 7805 snag is that you may want to draw an amp or more, and the drop from 12V to 5V (which is 7V) would result in the 7805 needing to dissipate 7W of heat. That's fine with a heatsink. Or you can put a physically large resistor in series with the input. A 4 ohm 5W resistor take up 4V of the drop (at one amp) and more than half the heat.

A less wasteful (of electricity) solution would use a 12V-to-5V switching power convertor module. See places like Jameco or Digikey for examples.