How do you convert 115AC to 12DC?

With a AC to DC converter.

You'd need to use a 115 AC to 12 volt DC converter. Connect the wires from the item you want to convert to the 12 volt positive and negative on the front of the converter. Plug the converter into a wall outlet and then turn on the switch on the converter. 115 is going into the converter but 12 volts is coming out and into the item you wanted converted.