Hi Mike and welcome to the forum, as Andrew mentioned, there really is a huge amount of research needed before buying the plywood. Personally, I tried to cover all the bases by making sketches and even making a small mock up of the layout out of card. That's because it's so hard to visualise how everything will come together. Have read about so many layouts that had to be heavily tweaked at a later stage. And sadly, some that were abandoned because they didn't work out as intended.
I spent so long trying to figure everything out before building baseboards, that I almost gave up. This damn fine hobby requires so many different skills and abilities. For me, I dreaded the electricery but by reading everything on here, plus watching countless videos online, I managed it.