Today I installed the second piece (of 3) on both of the lowest planks. It really does get easier.
Using a narrow piece of luan, dividers, a pencil and a straight edge, I marked off the plank. Then I transferred the marks to the planking material (9mm meranti plywood):
The plank was cut out and fitted (and refitted numerous times) to the frames & stringers on the starboard side. When I was happy with the fit, I flipped the plank over and clamped it in place on the port side. It was very satisfying to see it fit just right! So, using that piece as a pattern, I traced and cut out the second piece of lowest plank for the port side.
After lunch we - I had help - installed both pieces. I'm quite pleased with the results.
Tomorrow I plan to mark off, line off, cut, fit and install the last two pieces to complete the two lowest planks. These may be tricky as the ply bends and twists in two (or is it three?) dimensions but others have done it so I know it is doable.
Today's work was easier than those first two partial planks earlier in the week.