I get the batting folded into fourths. Then I fold the backing into fourths. Then fold the quilt top into fourths. Line up the middle marks on the top and backing first. Cut the backing about 4 or 5 inches bigger than the top, leaving it all folded into fourths. Then line up the middle of the backing with the middle of the batting. Trim the batting to about the same size as the backing. This is how I do it all the time. BUT I normally make quilts no more than twin size.
Otherwise I try to buy a batting that's around the right size. I don't have a big table where I can get it all flat and spread out. And I certainly cannot get up and down from the floor. It's been working for me.