I use a double bed for lap-sized quilts. I iron the back and use spray starch or sizing to insure that it isn't wrinkly in any way, Then I pin the back to the bed covering (quilt or bedspread) only at the top edge and smooth out the rest of the back before adding backing and top. I slide an old cutting mat underneath, starting at the center and pin baste there. I do move the mat around underneath until I have the whole thing pinned. For bigger quilts, I usually use large tables at our historical society or public library.