i don't like to cut the corners. it's too easy to snip off points or other design elements.
i used to fold in half on the bed. i checked with the largest square ruler i had, 12". wherever it was too full i pinned first and then took in tiny amounts on each seam until it almost laid flat. i didn't kill myself on this part. where it was too tight, i let out tiny amounts on every seam until it laid really flat. having a little bit too much is better than having a little too little. too much can be eased in, but too little can't and you'll wind up 'gathering' the border or binding.
now i have a big, high table so i don't have to use the bed, but the process is basically the same.