Here ya go. Just imagine that light bulb is your fan. And instead of a light switch, install a 12V switch of your choice (available at hardware or auto stores). Switches are about $3 unless you want something fancy.
Run one wire from the negative side of the battery to the fan. Run the other from the positive side. Cut one of the wires and splice the switch in between the wires you just cut. Viola, fan switch. Add a 10a inline fuse ($5) so you don't short something and fry your electrical system.