Step 3: electrical
IMAGE 1 - get two of the LR1130 button cell batteries. stack them, top to bottom. place an insulated wire, ends stripped, on top and bottom. DONT USE BARE WIRE! YOU RISK SHORTING ALL OF IT, AND RUINING YOUR LIGHT! wrap it in electrical tape
IMAGE 2 - on the left should be the stack of two batteries from image 1. run the wire coming from the top of the two to the bottom of the third. run another wire from the other end of the battery, and wrap it in electrical tape.
IMAGE 3 - very messy, so here is what you do:
1) run a wire from one of the switch terminals directly to the LED.
2) run one end of the battery pack you made to the other terminal of the switch, and the other end of the battery pack to the other LED connection.
3) flip the switch. if LED does not light, check wires to make sure there are no wires crossed anywhere. if still no light, swap the battery pack connections around so it is reversed from how it was. LEDs only light in one direction.
4) when all is correct, fit it all inside the lego brick. the batteries only fit if they are not all stacked.