NMJH, I used dress making pins for the first layer of planks, the ones with the coloured plasic blobs on the ends.
They are hard to put in by hand, so I used a pair of pointy pliars to put them in with. Make sure you scrape off excess glue on the ribs from the last plank you put on, sometimes some can ooze out, harden and make the next plank sit a bit proud, you need the plank to lie flat on the rib.
Sand the planking smooth before second planking, it dosn't need to be pretty or perfect, all will be hidden by the second layer, but it does need to be smooth.
Fot the second layer planks I bought clamps, they needed a small hole drilled into the first layer planks and ribs or they could split them. Make sure you put the clamps in where the rib is so it gets a good grip. The advantage of using clamps is that there will be no holes in the top layer of planks.
It is even more vital that you clean off excess glue when second planking, you don't want planks sitting proud, gives you extra work sanding down!
Good luck, take your time, and be ready to do a lot of sanding when finished!