Assembling our Achilles with wooden floor boards was half the reason we bought an air floor dinghy the next time around!
Richard has given you the order and the procedure, but a few more hints. The boat should be barely inflated. There should be two grooved ("U" shaped) side rails, probably made of aluminum.
Don't put the side "u" rails in until the very end, and don't inflate the inflatable keel until the very end.
You will find that the fit is very tight, so when you put the front piece in, and the back piece, the one or two middle piece(s) will seem not to fit. They will, if you tent the boards a bit and then push down on them. Remember to have those "H" pieces between each board. Then after the boards are snugly in, take the "U" side pieces and slip them onto the sides (sort-of roll them down and around). Then inflate the boat fully, side pontoons first, keel last.
You are not alone in the frustration of trying to get the entire thing assembled, but you will be rewarded with a sturdy dinghy.