The folding starts with a square sheet of paper, but unless you're an origami aficionado, you probably only have the ordinary rectangular sort around. No problem; you can easily make any sheet of paper into a square sheet accurately enough for this build. This works best with 8 1/2" x 11" or A4 printer paper, but I've made these gliders out of note pad sheets as small as 4" square (after squaring up) and as large as tabloid (11" x 17" or A1); the smaller ones don't glide as well (too heavy, because the paper thickness doesn't scale with the sheet size) and the larger ones are fragile (again, paper thickness doesn't scale, so you wind up with thinner paper relative to the forces of handling and flight), but with care, they do work.
To demonstrate squaring the paper, I've started with a common letter size sheet.
Wonderful! :)