It can certainly be difficult when the quilt is larger than your available flat space! What I do is call my local church office, find out when one of the very large open areas will be completely vacant, and do it there. They're very nice about it. I take the quilt top (nicely pressed), the backing fabric (nicely pressed), batting, a roll of masking tape, lots and lots of pins, a couple of hand-sewing needles, thimbles, scissors, and a couple of spools of "I don't like this anymore" thread.
As to lining everything up with making the quilt sandwich, I spread out the backing fabric, right side to the floor, tape it down with a bit of masking tape to make sure it stays completely smooth, and then spread the batting on top of that. I line up one edge of the batting exactly with the backing fabric, pin it in place on that side, and smooth it out from there. Finally, I take the quilt top and line up one edge with the "lined up and pinned" edge of the backing-and- batting, but set it so that the quilt top edge is about 1 inch in from the actual edge of the backing-and-batting -- an inch of batting-and- backing will show. I spread it out carefully from there, and then pin it here and there. The edges and corners of the quilt top do NOT meet the edges and corners of the batting and backing, since both the backing and batting are a bit larger than the quilt top itself. Thus, you see "extra" all the way around. After adding quite a few straight pins all over, I remove the masking tape, gently lift the quilt sandwich, and flip it over to be absolutely sure there are no wrinkles or pulls and that everything is smooth. Then I flip it over one more time so the quilt top is on top, and baste -- and baste -- and baste. If the batting-and-backing is more than a couple of inches extra all around, I trim it so only an inch or two extra is left. I don't trim off all of the extra batting and backing until I have finished the quilting and am ready to bind the edges -- this keeps the edges of the quilt top from getting a bit raggedy. This system works for me.