Welcome to the word of nibbling straking!, by Firstmate
(Try saying that three times fast!)
Hard to explain, maybe even harder to do. But I'll try.
Method A - Template method:
Make the margin plank about 1 1/2 times the width of the deck plank. It may be hard to bend around at the bow, so cut the plank to fit the curve then scarf that in to the straight piece. The nibbling cuts are made as you go. When the taper on the end (snipe) gets to be the length of twice the plank width, you start nibbling. Lay the plank end in place over the margin plank, and put a witness mark where the plank crosses over the margin on the outside. Put another witness mark at the end in the middle. With a ruler, join the two witness marks and cut. This template is for cutting out the margin plank. Use a sharp scalpel blade and a small razor sharp chisel.
Method B - Cut and pay in as you go:
This method is very much the same as the one tile and flooring installers use to cut around floor and door moldings and such . Where they lay the piece to be cut over and on top of the preceding piece, then layout a cut mark. As you can see in the diagram below, it's basic geometry:
Lay the next plank over the Margin plank (Covering board) and mark it's center (A) and the end of the prior plank (B).
Then, cut the plank along the resulting line (C), lay the plank back in place and use the new edge and end to mark the "joggle" in the Margin plank.
Cut out the joggle to fit the plank - when satisfied with the fit, attach this plank and proceed to the next one.
As always, practice on scrap pieces before working on the real thing. And always cut to the proud side of the witness mark, then sand down to fit. It's a slow process with no shortcuts. No two will be the same, even on opposing port and starboard sides.