Start by laying out your batting on a clean, smooth floor. Spread your quilt top on the batting, smoothing out any wrinkles. You may need to actually crawl on the top of the quilt to do this. Trim batting to within about two inches of your quilt top.
Starting from the top, carefully roll your batting, as shown.
Continue rolling until you have a batting tube like this and set it aside. Don't worry about pinning the batting roll. The natural tendency of most battings is to cling to fabric, so your roll should hold itself together without any help from you.
Now spread your quilt back on the floor, right side down. Starting at the bottom of the quilt, use a strip of painter's tape to secure the edge of the quilt back to the floor. Move to the opposite (top) side and, pulling the quilt back ever so slightly toward you, tape the center top to the floor as well. Repeat with the left and right sides and each of the four corners, each time pulling very gently, but not stretching, to make sure the quilt back is completely smooth.
Bring back the batting roll and, starting at the bottom, slowly unroll the batting and top onto the quilt back. You have several inches leeway on all sides, but want to make sure that a) all parts of your quilt top are "inside the edges" of the quilt back, and b) your rows are perpendicular to the sides of the quilt back and parallel to the pieced row on the quilt back. If you see that you're off, don't hesitate to re-roll your batting and start over.
Depending on the width of you top back fabric, you may have several extra inches at the top of your quilt. Don't worry about that for now.
Once again, smooth out your quilt top and batting. I usually do this by starting at the bottom and crawling up the center of my quilt, smoothing as I go. You want to make things smooth, but be careful not to warp your fabric as you work. If you notice that your smoothing is making your blocks wonky, ease up a little bit and work them back into a nice gridded shape.
Once your fabric is arranged the way you want it, use curved safety pins to secure all three layers at the center of each block. If you're new to free-motion quilting, you may also want to place pins at the intersections of the sashing and around the edges.
Placing these pins is awkward at first, but it's something you're likely to get the hang of with practice!
Once you've placed all of your pins, use scissors to trim through all layers about two inches from the edge of your quilt top.
You'll want to handle your quilt sandwich with some care. However, if you've done a good job with smoothing and pinning, you should be able to flip your sandwich over and have the back be as smooth and even as the front.