Make sure your backing is larger than your quilt. Iron and starch it very well.
Lay the backing out right side down on the floor or a large table.
Tape or clamp it down tightly.
Smooth the batting over the backing. (The batting should be larger than the quilt top, too.)
Center the quilt top on the batting and backing.
Pin every 3 - 4 inches all over the quilt starting in the center. (I was told a good way to check your pin width is use the width of your fist.)
If you don't want to pin baste, many folks use spray basting or thread basting. Whatever method you choose, be sure to baste it well.
Quilt as desired. A walking foot is very helpful.