Is it flat boards? If so, put a small amount of air in the tubes. Then put the bow floor in and the stern floor. Then put the middle bits in with the edge where they meet raised up like this
bow __/\___ stern (seen from the side)
Then lean down on the raised joint and the boards should snap into place.
Take care not to trap the tube.
Then put an oar or something else straight under one side to give you a level slightly raised surface. You can then hold the boards flat and slot in the stringer. Once the first stringer is in place, the one on the other side goes in easily.