To keep things simple, I decided to use miniature PCB-mounted relays to select the cameras and, as there were no complications regarding keeping the video sources "in sync," the circuit could be kept really simple.
The circuit is shown on the left but we need not concern ourselves with how it works - all we are interested in is how do we get from this circuit to a fully functional piece of hardware? Exactly the same principles we use here will apply to any other circuit that we may find in a magazine, for example.