I join the sides together to form one solid chunk or side piece. When done ...assemble them with the end pieces (bow and stern) , then the ribs in the top section , now for the bottom. The bottom of the ribs are flush with the two side panels so they fit the bottom of the boat.
Next step is the outside rail before you try moving the boat , without the outside rail then the sides might buckle (when you move it ) and break the ribs on the boat.
Now attach the bottom . the sides might have a slight wave in them but the inside rail will get rid of that , when you attach it.
One easy way to do it .... When you have the sides done , then place the boat on the bottom and CAREFULLY .... mark the outline of it on the bottom piece without moving the sides. Use a pencil , lead one not ink.... :roll:
Now draw a line about two inches out from that and use that line to cut out the bottom chunk to fit the boat.
Attach it the the sides and when cured then carefully saw off the excess wood to form the bottom to the sides.
Chuck.