It's always tricky to start with on the big pieces, balancing the need to fold it to get it through the machine with the desire to cause as little movement in the quilt top. I generally start with the most fragile bits, rather than moving from the middle to the edges, trying to keep the quilting even over the whole quilt. One way to do this is to quilt with one colour thread first, in this case I used dark brown to stitch the outlines of the top layers of brown - this holds all the layers below the brown in place as well.
The problem is that sometimes at the start it can look a bit messy, with threads around and little bumps, but it usually comes out fine. I've got the first bit done, and now I'm moving onto the more fiddly bits knowing that layers will stay where I put them.