Choose a large spot on your floor - hardwood or tile is better than carpet. Then clean your floor! This is very important, especially if you're using a white backing. Also make sure your feet are clean - you're going to be walking and crawling all over the quilt. :P
(Also, shoo all animals out of the room, otherwise picture 2 will happen.)
Iron your backing and lay it out - smooth it out so you have no creases or bumps.
On top of this, lay out your batting. Smooth it out as well, and make sure it's centered on your backing.
Now, lay your quilt top down. Make sure everything is centered and smooth it out.
The best way to smooth it out is to get down on your hands and knees, honestly. You can either go end-to-end or from the middle out. Do what works best for you - just be sure to be very thorough!
Once it's smoothed out it's time to pin it!