Solder three LEDs in parallel with a resistor between each of the LED's short leg and the ground wire. To learn more about soldering, check out this Instructable
and to learn about LEDs in parallel, check out this one
. We're using solid core wire so that we can position the LEDs into a standing "tree" once we've soldered them. Make sure your wires aren't too tall for your dome!
Solder the power and ground wires to the battery pack's wires and, using electrical tape, secure the LED "tree" on top of the case so that the LEDs stand straight up. Turn on your battery pack to make sure your soldering worked.