Your drain lines need to be just below the level of the water in your sump. 1" below is optimal. The ensures a fast take up if the siphon needs to be restarted (if the power goes out and back on, or you shut the return pump off and restart it).
Once you have the set, then leave all 3 gate valves wide open. Turn your gate valve 1/4 closed on the return pump line. Don't be scared of it overflowing, this system is designed with that possibility in mind. The first time I turned mine on, I thought for sure it was coming over the edge of the DT, but you will get used to how it works and never have another worry again
Now you will start to fine tune the siphon/return together. I tune mine using the siphon channel gate valve. once you have that set, then see how much flow you have going into the tank from your return lines. If it's too much, then you will turn the valve to restrict the pump a little, and close the siphon line valve a little. If it's not enough, then open them both a little.
The Return and Siphon valves are directly related. You adjust one, and you must adjust the other the same way. it's a bit of a PITA at first, but you will get the hang of it.
You will also notice that the water level in the overflow will even out too. I run mine right at the bottom corner of the elbow, this way 99% of the flow is going through the siphon channel.
Also make sure that your open channel air line is just above that level, this way it doesn't fight to be the siphon.
It takes some time and patience, but once you have it set, your going to be a happy camper.