When I found this method, I tried it, and it was like the heavens opened up and a choir sang, "AAAAWWWWHHHHH!!" I showed my guild; I was ecstatic. I've used it on every single throw-size and up quilt I've quilted on my Elna and then Bernina ever since.
Here's how you do it:
1. Tape your backing down to the floor as per usual.
2. Lay down the batting, again, as usual, except do not spray or pin baste yet.
3. Without spray basting anything yet, lay down the top, smoothing it out as well.
4. Divide your quilt top in horizontal thirds in your mind.
Tip: I've found that not all quilts will divide at a logical starting and stopping quilting spot, so sometimes the thirds are uneven. For example, you might have 24" at the top and bottom, and the middle section 36", just because of where your quilting motifs might land. For my "Brrrr Park" quilt, after much pondering, I've had to go with dividing my batting in half, since in the two huge open spaces, one between the top row of trees and the middle, and the other between the bottom row and the middle, I plan to quilt huge feathered wreaths. If I cut the batting in thirds, i.e. below each of the 3 rows of trees, I'd have two issues: one, I'm cutting the batting apart in a focal area, and two, I would have to quilt each feathered wreath half at a time, not a good idea for flowing quilting lines.
4. Once you have your top lined up and squared up, carefully fold back the top 1/3 and bottom 1/3 (or in my case, 1/2) of the quilt top, onto the centre 1/3 of the quilt top. Mine is folded back at the halfway point.
The reason why I lay out all 3 layers without basting is to be sure I have all layers squared up, as well as to be sure I cut the batting in a logical place. Had I merrily cut my batting in thirds without laying down my top, I would not have seen the issue with needing to quilt my feathered wreaths in one go.
5. With scissors, carefully cut apart only the batting layer at the desired spot. I use a wavy line as opposed to a straight line for three reasons. For one, it's easier to match up the cut-apart section once you have finished quilting the centre section, and two, it also ensures that various lines of quilting will cross it (i.e. you might be doing cross-hatching in one area, straight-line in another, free-motion in another, and were you to be doing a straight line it wouldn't follow right along a straight join) and three, the cut-apart edge is easily disguised or hidden. That said, I've never had a cut-apart edge visible or "feel-able" in any quilt and I've been doing this method for over 15 years.
6. Using a permanent marker, or a piece of masking tape and sticking it to each section, label the top third and bottom third and right or left edge of the batting you've cut away. Be sure to write on the excess edge of the batting!
7. Remove the cut-apart batting. Smooth the centre area, and either spray or pin baste as you normal would. I still do not move the top layer so that I will have it positioned correctly. Fold back half of the centre section, baste it as you normally would (see here for how I spray baste), and then repeat the process for the top half of the centre section.
8. Below you see me doing just that: folding back the top on itself so I can spray the top of the batting. I've already glued the backing to the batting.
9. Repeat with the other half of your centre section. QBS is so wonderful because you can reposition multiple times so you get all layers nicely smoothed out. Here I have my top half all basted.
Pull up the basted quilt once you are done. Remember to flip your quilt over to the back and gently smooth out the backing before you start quilting.
This revolutionized my quilting life because I could easily do more intricate designs in the centre of the quilt since there was 2/3 less bulk to handle, a huge deal when you only have 7.5" throat space!
Quilt in the ditch to stabilize the quilt. See how the quilt top and backing easily bunch up on the table behind my machine, since there is no batting for half of this quilt? With the Quilting in Thirds method, you have 2/3 less batting to manipulate for the centre section!
Once I have finished quilting this section of Brrrr Park, I will tape down the quilt once again (that is the only downside to this method; you have to tape it down three times). I'll line up the cut-apart batting, matching the wavy line, spray baste the layers, pull it up off the floor and continue quilting, carrying on with the stitch-in-the-ditch along seams first. In the book, she recommends whip-stitching by hand the two section of batting, which I used to do. I find it's not necessary especially if I'm doing fairly heavy quilting, because the many times my stitching crosses those cut-apart edges will hold everything together just fine.
I love this method because you only have to deal with the entire bulk of a quilt for the last third of the quilt!