This part of the build is time consuming. I draw out the spar on the luan. For the ribs I cut paper patterns and glue them to the luan. Next, I cut out the ribs and spar with my bandsaw.
I check all the ribs against the full size drawing to make sure they're the right lenght. Then, I check the ribs against the spar to dial those in. I like to make sure the ribs height is the same as the spar for that rib location. It also gives me a chance to check the rib profiles.
I like to draw out interior sections of the ribs and spar where material will be removed. It saves a lot of unwanted weight by doing this. I drill a hole or two in those sections. Then I use my jigsaw to cut it out. I've found it handy to build a jig that gives the spar and ribs some support when making these cuts.
I hope this helps anyone making a wood surfboard. Thanks for your support.