I fold the backing, batting and top and mark with safety pins. I line them all up I also mark the center - wrong side on the backing, right side on the batting, I make sure the centers are even before I remove the pin on the batting. I then make sure the top and backing pins are lined up. After I pin them together, I take out the pin in the backing.
I have also lined up the border onto the backing and worked over to the other side and match those. I recently did this on two baby quilts I made. I try to avoid all of this by adding fabric down the center or use the same backing fabric added to make the back larger.
I try to use backing fabric, this is the easiest.