My Greenland style skin on frame was built using the "Sea Kayaker" instructions. I didn't have to install the keelson first and then add the ribs. The ribs were cut to shape and numbered for installation. It want as follows (or something thereof):

1.) Lay the rib stock perpendicular to the bottom of the gunnels at each mortise. 2.) Measure the length of each rib from gunwale to gunwale and add 8-finger spans to the length of the rib. 3.) Cut each rib and number it to its corresponding position. 4.) Now the tricky part (this adjusts the rocker and reduces volume in the stern) (I don't remember the exact measurements the instructions used):

Butt all the ribs (in numerical order) against a piece of wood or T-square, so that the ribs form sort of a bell curve. With rib # 1 (bow) at base 0, measure 4" or so up on the last rib (stern). Draw a line from rib #1 to rib #x and saw each rib shorter so that it corresponds with the line you just drew. 5.) Steam and install.

Hope this makes sense. I fine tuned the rocker by shimming the keelson with blocks of wood at the stern.

Lew