You can use 12 volt light bulbs in a 6 volt system easily, but it simply means the light bulbs won't be quite so bright. However, if you're using more than one light bulb, you need to wire them in parallel, rather than series. Wiring in parallel maintains the same voltage throughout the system, while wiring in series means the voltage gets reduced...
M/F Select the number of 12 volt light bulbs you want to use in your 6 volt system. If your system is battery powered, the more light bulbs you use the quicker the battery will go dead.
M/F Cut strips of AWG 18 gauge wire using a knife. The number of strips you need to cut depends on the number of 12 volt light bulbs you're using in the 6 volt system. Multiply the number of light bulbs you're using by 2. For example, if you're using 4 light bulbs, then you need 8 wire strips. The length you cut depends on the distance you want between the light bulbs.
M/F Strip about ¼ inch of outer plastic from the ends of all the wires using wire strippers. Loosen the terminal screws on the light bulb holders using a screwdriver.
M/F Insert the end of one strip of wire under a terminal screw on the light holder, and then insert a second strip under the other terminal. Tighten the screws.
M/F Insert the opposite ends of both strips of wire under the two terminal screws on the second light holder. Insert the ends of another two strips of wire under the same two terminal screws on the second light holder. Tighten the two screws. You now have two wires extending from each terminal on the second light holder.
M/F Connect the opposite ends of the two unattached wires from the second light holder to a third light holder, if you're using more lights. Repeat the process by attaching another two wires to the third light holder. Once you have connected all the light holders you want to use, connect the last two wires extending from the last light holder to the power source and your 12 volt lights now operate using a 6 volt system.