As a beginner in woodworking I would like to build a simple box from plywood. The edges will be simply glued butt joints which means they need to be straight and smooth. When I use a hand saw I can't quite get it to go along the line and it can veer off a bit. I can compensate by sawing a bit further from the line but sanding down the excess seems to take longer then would seem reasonable.
Given that the cut lines I drew are accurate, what is the best way to accurately cut a rectangle or square from a plywood sheet with unpowered hand tools? Are there other tools I will need beyond a hand saw and a sanding block?
To cut the plywood I am working with a two sided Japanese pull saw. Similar to the following picture.
For short lengths I can make reasonably accurate straight cuts. The problem occurs when I want to make a 50cm-100cm straight cut. I have a lot of patience so speed is not an issue. Mainly I am asking how people did this sort of work before they had powered tools. Am I using the wrong saw? Should I be using guides? Should the saw just be used to get close and a wood chisel is used to remove the small excess?