A normal single throw relay should have a diagram like this on it:
Its very simple how it works, pins 85 and 86 turn on an electro magnet which in turn pulls a switch shut, connecting pins 87 and 30.
So (assuming the bulb works as I would expect, being on when positive is applied, as opposed to being constantly live and negative applied to turn it on) you would splice the bulb cable and attach it to pin 85 and then 86 to ground.
If the bulb is constantly live and activated by a negative, then you would connect pin 85 to it in the same way, and pin 86 to a ground in the car.
You would then attach pin 30 to a constant positive (battery terminal, and other constant +ve feed), and pin 87 to the "remote on / power supply" for your reversing camera.
In this way when the reverse light turns on, it flips the switch turning the reverse camera on, and when you turn it off the relay turns it back off.
If you find it works, but the bulb no longer illuminates at all, then you need to splice the other side of the bulb connector as you're making the circuit cut out the light. In most cases it shouldn't matter which side of the bulb you attach the relay (depends on the resistance).
I hope my description makes sense