Make sure both chambers in the OF are full of water. this means that both ends of the u-tube will be submerged. Place a flexible pc of tubeing in the u-tube. The end of the tubeing should be at the peak of the u-tube. Put the u-tube in place. Now suck out the air in the u-tube. The water should come up from both sides of the OF. When that happens, pinch off the end of the tubeing that you're sucking on. This will not allow air to get back in the u-tube. Slowly pull the tubeing out of the u-tube while keeping the u-tube submergered in the chambers. Turn on the pump and it should start to drain. If theres a little air make sure that the water carries it out, if not try again so there is no air in the u-tube.
Once you get the hang of it its not a biggie and don't worry you'll get a mouthful of SW before you get good at it.
HTH
Tim