1. Cut a hole through the washer where you want your switch to go. Make sure it's large enough for your switch to operate smoothly.
2. Solder up and test your battery holder, on/off switch and LED setup. I can't tell you exactly how to do this step because I don't know what you are using. Take a look at my insides picture to see how I did it.
3. Hot glue the battery holder, switch and LED to the posterboard.
4. Cut a piece of aluminum foil a little bit larger than the arrow and hot glue that down to help spread out the LED light.
5. Put some hot glue in 2 of the seams of the posterboard to make it a stronger box. Don't glue it all together because you will need to open it up to change the batteries.
6. Close it up with some pins or however you want and hang it up.