I use the back of a sandbox to place down canvases, as it's super large and perfectly flat. (plus it only costs 500 Units!)
I place the sandbox on the back of a canvas, on the very edge (regular ones, not the primitives shapes).
about here :
Then you put another canvas next to it.
If I'm building a floor, I also put a snap grid (pressing E while dragging the item) of size 10 to perfectly align them.
If I'm building a wall, I generally don't use snap grid. (it can be inexact sometimes)
Also, be careful with regular canvases if you're using snap grid!
You'll have to have them rotated the same way to align them perfectly, as the X, Y and Z axis arrows aren't placed at the exact center of the item.
If I'm building a roof or a ceiling, there I use canvas cubes, as placing down regular canvases upside down (and any item) is terribly wobbly (idk why).
To build a roof or a ceiling, simply build it as if you were building a floor, but with cubes!
(put the Z axis of the cubes at minimum so it doesn't take much space)
Hope that helps.