I use my king size bed to lay it out on, folded in half. Then I can mark with a ruler & marker first, where the 'off' area is. I will cut with scissors or put my mat under the edge and cut with rotary cutter.
I have also folded into 1/4's and used the same method. But you do have to make sure your folds are perfect and even, and then it will work fine.
I cannot get on the floor at all, so this is the way I do it. I also will lay it out over my longarm frame to check it to begin with - also allows me to quickly square up backings. This at a minimum, let's me know how far off I am, if at all. And it's quick and easy. I still have to move to the bed sometimes - or else I trim on the frame (I have a wooden top that allows this).
Just what I do...
Debbie in Austin