Here's the basic "suck out the air" procedure.
Get yourself a length of airline tubing, 3-4 feet long works pretty good.
Return pump off - obviously.
Sump should have "some" saltwater in it - like around 1/2 full or so.
Fill the outer box until the saltwater stops running down the drain. A properly designed overflow will allow some saltwater to remain in the outer box.
Pour some saltwater in the inner box too.
Place the siphon tube in the overflow. There must be saltwater covering both open ends of the siphon tube. Must.
If so, remove the siphon tube again and slip one end of this airline into the siphon, and push it up there until it's open end is just a little past the highest point of the siphon tube.
Hold everything so it doesn't move around on you, and return the siphon tube to the overflow.
Both ends of the siphon are now under saltwater with this airline hanging out a couple feet.
Slide the airline out a little so the end that's in the siphon is at the HIGHEST point in the U tube siphon.
Now - suck the air out of the siphon tube by sucking on the airline with your mouth.
A spit cup is handy right about now - if this is your first attempt.
You'll notice as you draw the air out, that the saltwater in both inner and outer box begins to raise up in the U tube. When you get all the air out, pinch off the airline tubing with your teeth or fingers and pull it out of the siphon tube.
Keep the airline pinched off while you're doing this or air will rush back in through the airline tubing and you'll lose some of the saltwater in the siphon tube.
Once the U tube is filled - you have now "primed" the siphon, and it should be safe to turn on your pump.
If this is your first time doing this, you may want a couple few gallons of saltwater mixed up on hand. You may notice a small drop in your sump or wet/dry, and will need to add some more water. How much all depends on each person's set up.