Hi Bazzg,
I assume this is a double planked. for first planking It does not matter if you start at the top or bottom. The idea is to get a good hull shape to put the second layer on. I suggest that for ease you start at the bottom and plank along the keel first. Take your time doing one plank a time either side of the hull. You wll need to bend & cut the planks into position. They do not need to go the entire lenght of the ship in one go. You can cut them mid bulkheads. It is easier to do smaller sections and shape the smaller the planks to fit. Tying to do multiple bends in a single plank is not easy.
The planks will bend if soaked in hot water for about 20 min. Shape them, bend into place and pin. Do not glue, wait till the planks dry and then refit and glue them. The should keep their shape when they dry. Planking IS the hardest part and takes time to learn. Do worry if its not perfect. As long the shape is there is filler and sanding will fix the rest.
It took me 5 ships to get in the swing of it. Take your time, My santa ana took 6 months to plank.