I think you're on the right path. I simply cut the micro USB cable off of the stock charger, ran them to each of the four camera locations, and tied all the cut ends together at the power supply's output (with the ammeter placed between the PS output and cameras' leads).
So, in your case, I would say just cut the large end off of your cable. You'll find a red & black (obviously + & - 5VDC) and green & white (data). Since data is not used, you can snip those off.
FWIW, even though these cameras don't pull much more than 500ma under the worst case scenario, that is enough to cause a voltage drop across long, thin conductors. I don't know what gauge your cables are, but I've seen some that are VERY thin. And driving a 0.5A load, which is only 5VDC to begin with, over say, 12', can cause it's own problems. The PS may be more than adequate, but if the cable is too thin and long, the electrons can't get through fast enough, and you end up with something less than 5V at the camera.