Make sure you over flow is in the water and both the front and back part of the box are filled with as much water as it will hold. ( the box will not be completely filled bc it will keep draining over to your sump). So fill it until the water starts to run in the back of the overflow box into the sumo and stop. Im not sure if you have the
U-tube or a different box but start the siphon. You will want to do this first so that you tank doesnt overflow on the floor when you turn the pump on.
Next to add the correct amount of water:
Its pretty much a trial and error type of thing. So fill the sump where the water is going over the baffles into the lowest chamber with your return pump. After all chambers are full to there max add more water until its 1in over the highest baffle. The water level will be even on the sump. Turn the pump on. This is where you will need to use trial and error method to adjust the water level in the return chamber. You may needed to add more if the water has dropped to or below the return pump or take water out if the water is not running through all the baffles correctly. This is after the pump is on and water has started running from thr tank to the overflow.
Next after the water level has became constant turn your return off and make sure that your sump doesnt get to much water that it floods of the sump. If it does raise the height of the overflow box. If it doesnt you are good to go and start you pump back up.