Dakotamaid is totally right!!
Tape a toothpick on each END of your dining room table in the middle of the width of the table (to mark the middle point).
Fold your backing in half to find the middle and position it precisely onto the toothpick. Match the middle of the quilt back (at both ends of the backing fabric) to the toothpick. Put the right side down, of course.
Let extra fabric fall on either side of the table.
Now put your batting onto it.
Now put your quilt top onto it. Match the middle to the toothpick.
Each time, for each layer, make sure the middle is matched to the toothpick for each layer.
Smooth out, pin with lots of safety pins, or baste with extra large stitches. Move from the center to the sides. It's very easy.
You have sandwiched your quilt -- ready for quilting it!!