This is my technique for layering a finished quilt. I found that trying to get all the layers to lie flat for pinning and/or basting was difficult. I've made many quilts and have found this to be the best technique for keeping all the layers flat and taut for machine or hand quilting.
Step 1: Lay Out Backing Fabric
After sewing together your backing fabric, lay it down on a flat surface (right side down). I used to use my dining room floor because it was the flat surface in my house that was large enough. When we put in a new tile floor in my laundry room, I found an additional surface that works. Make sure the fabric is square with the floor. The tile blocks work well for this as do the lines on a hardwood floor.
Step 2: Tape Down the Edges
Using the lines on your tile or hardwood floors tape down all the edges of the backing fabric. I used wide masking tape, but have also successfully used packing tape and blue painter's tape. Make sure the fabric is taut and square. (test the tape if you're using a hard wood floor to make sure removing the tape doesn't ruin the finish on the floor).
Step 3: Layering
Spread out the batting onto the backing. Smooth out all the wrinkles. This is where the backing would start to shift and wrinkle if it weren't taped down. Then put the finished quilt top on the batting. Make sure your batting is larger than the quilt top.
Step 4: Lots of Pinning
At this point you could baste the layers together, but I prefer pinning. You'll need enough pins to put one every 4-5 inches all over the quilt. Try to avoid placing the pins where you'll be stitching. I like the bent quilters pins because they are easier than standard safety pins. You can purchase these pins at your local quilting store or at JoAnn's etc.
Step 5: Finished Quilt
When you're all done with your hand or machine quilting, remove the pins and finish the edges.