There are two ways to do the water jugs.
The other way, that they use in the movie...
Fill the 5 gallon jug. Pour 3 gallons from the 5 gallon jug into the 3 gallon jug, leaving 2 gallons in the 5 gallon jug. Pour out the 3 gallon jug. Pour the 2 gallons from the 5 gallon jug into the 3 gallon jug. This leaves the 5 gallon jug empty, and 2 gallons in the 3 gallon jug. Refill the 5 gallon jug, fill the last one gallon to the 3 gallon jug leaving 4 gallons in the 5 gallon jug.
The real issue here is the jugs are clearly not full and they spill a bit, which would not be a problem except Simon said not to be off by even an ounce. I suppose it may have been hyperbole, or just BS.