If you're cutting loose stock lay it over the plans, lay a small metal straight-edge over the part and sight straight down on it and use a pensil to scribe the line. Then cut it with a razor saw. I have a very square cutting block (it the end of a Butcher Block countertop) and just by hangint the piece over the edge with the pencil line aligned visually with the edge I can saw through it very perpendiculaely (but then, I used to work in a cabinet shop and have done a lot of "full-scale" handsawing).

You can also saw close and then work it perfect with 60 grit sandpaper on a sanding block or laid flat on a table (never with loose paper held in one hand).

If you're transferring an angle from a harder to manage sub-assembly, invest in a "T-bevel" (the brass & wood tool on the board in the lower right of the first image). It adjusts to an angle with a wing nut and then locks in place.

While you're shopping check out a Try Square (the wood and steel right angle in the second image) and a 12" spirit level (the guy with the bubbles). These three tools are useful in ANYTHING you will ever build; bookshelves to biplanes. Level your worksurface with the spirit level and then you can use the try square to check squareness from then on. Use it to mark stock, check a stack of wing ribs when sanding them to shape , etc.