when i lay my quilts out, i do it on the living room floor.
if it's a large quilt, i fold it in half, then lay the muslin, usely in half on top of it to see how much to cut off the bolt. mark it with some extra, fold the quilt top out of the way, then use my rotary blade and mat to cut it. i measure my batting the same way, on top of the quilt top, measure, fold the quilt out of the way, and cut it off the bolt with my mat and rotary cutter.
then wash the muslin backing. material could be figured the same way. usually when ironing my top and backing there is a crease down the middle. i use that for a guide in laying out my quilt sandwich on the floor.
lay backing down, batting then quilt top on top. worry about getting the center down the middle straight along the crease from ironing, and make sure there is extra all aroundthe edges. 3-4 inches is enough. it does not have to be completely smooth and wringle free on the floor.
PIN WITH SAFTY PINS IN 3 PLACES ACROSS THE MIDDLE IRONING CREASE WHILE IT IS LAYING ON THE FLOOR. transfer the sandwich to a large table.
with you hand in the middle where the pins are, reach up and tug gently on the backing to straighten, and the batting, smoothing all 3 layers across the top of the table. i have used large fishing wts to help pull the 3 different layers smooth, but you don't have too. letting the quilt hang off the table overnight will help settle all the layers down. next day add loads more safetly pins to baste,