For your overflow. You adjust the box so that the lip of the overflow is below the water line (so it fills). Then the thing you have to do is get the water to flow through the tubes and over the back of the tank into the back chamber so it can drain. You need to remove the air so the siphon starts.
Take a small airline hose, shove it up the tube up the siphon tube and suck the air out. Good idea to have 2 people as you need the return pump running at the same time so the water level stays consistent and can continue to overflow and... well.... keep the cycle going.
Good luck. The first few times it can be nerve racking
Melev's Reef - How to start an overflow box