Hello Martyn
Firstly either find a set of plans at 1:35 or draw up your own basic plan showing length, breadth and height. If the model is to be used in a dirama and dipected at waterline level than you wont have to worry about keels, rudders and propllers.
The drawing up of a set of basic construction drawings is easy enough if you keep it simple. 
Then consider what you want to make the model out of i.e. wood (balsa / ply), styrene or card.
After you have answered these questions I will be able to help you with more detailed information.
Paul