Here's a couple of tips for those starting to plank the launch
If you're a little nervous about this and wasting planks while trying to get it right, just get some thin cardboard and cut it into strips the same width and length of the planks. Keep cutting with blade or scissors until you are happy with the shape and fitting. Then trace the around the card shape onto the plank. You will still have to sand but not as much.

Another thing you can do, and this will apply to the rest of the ship, save your sawdust and when you want to fill in gaps with your PVA glue put some on a glass or plastic surface and mix some of the saw dust in. This makes it ideal for sanding down.

Hope this helps.