Step 16: Connect external wiring
2) feed the power cable and the 6 wires from the LED's through the strain relief, with about 6 inches inside the box. tighten down the strain relief to clamp the wires.
3) attach the wires to the terminal block. use a multimeter to check that you've connected the LED positives and negatives correctly.
4) before connecting the power cable, it is convenient to test the circuitry using your bench power supply. you can test safely by slowly increasing the input voltage with a 250mA current limit, to check for shorts, miswiring, etc.
5) if you are using a laptop power supply, note that these power cables usually are coax - the positive wire is in the core of the cable with an insulator around it, and the negative wire is all around the insulated positive wire. that makes it a little tricky to strip correctly.