One thing Scissor Queen forgot to make clear is that you draw your line on the Diagnol-45 degree from one corner down to the opposite corner. Use the finest pencil or pen that you have and sew a Scant quarter inch on each side of the line.
On the ones you have already done, you might try starching the you know what out of them to give more stability before trying to true them up. Hope you can salvage them.