Hi Page.
When all your ribs are glued square to your keel you can then do the ply deck.
Take the ply pieces and draw a line straight down the middle one end to the other also lines across the deck at each cutout. Put glue along the top of the ribs and keel and place your first piece between the ribs. Line it up with the centre line you drew then pin in place down the middle.
Then pin the ribs in place one at a time ensuring you keep all lined up with the pencil lines.

*Advice*
HMS Surprise has got to be one of the hardest kits to build out of the Modelspace kits.
It isn't a kit that I would attempt without building a couple of kits before. I built The Bluenose and SanFrancisco II before mine.
No disrespect but I think you are struggling to build this one. Yes the instructions are not brilliant which in turn makes it harder to build.

I would recommend as a start to try HMS Victory which has the best instructions and video tutorial around. There is always the Sovereign of the Seas or the Endeavour longboat both having good instructions.
I would save the Surprise parts put them to one side and try one of the ones I have recommended.

Regards
Chris 😀