If you have not yet basted the layers together, I highly recommend heavily starching the backing fabric. I do this by painting on a 1:1 solution of Sta-Flo liquid laundry starch and water (using a large wall painting brush, so it is fast), tossing in dryer, then ironing with steam. I also spray starch the top before layering. Starch stabilizes the layers so there is less stretching and distortion while machine quilting.
If you have already basted the layers together, you can still stabilize with layers of spray starch. Spray starch, let dry (a fan can speed this up), spray again, let dry. Do this on both sides of the quilt sandwich.
If you are not doing to do free-motion quilting, then it helps a lot to use a walking foot.
I do not recommend starting by sewing all the diagonals, because these are on the bias. One of the problems with lines that cross each other is that these are the places where any stretching of the fabric will show up as a pucker or tuck. All of the diagonals in your quilt are on the bias, and the longer a sewn line on the bias the more likely there will be some stretching of the fabric -- which will show up when you start quilting lines on the opposite diagonals. Starch and a walking foot will help prevent stretching, but a first-time quilter may not realize that you still need to handle the sandwich carefully.
Ideally you would want to stitch on every block line across and up-down to stabilize the sandwich before sewing diagonals. However, stitching in the ditch (SID) is not that easy. What I recently did that I think would work on a Halloween quilt too is to SID with a fairly narrow zig-zag stitch instead of a straight-line stitch (using a colorful thread). This is a much more forgiving way to SID and is easy to do with a walking foot. Once all the block lines are stabilized, you could do all the diagonals. Once the diagonals are done, you could add lines in the plain blocks to the spider web veins, and then stitch lines to connect the veins. Because these would all be "wonky" zigzag lines, the spider webs would have wonky zigzag lines instead of straight lines.
Create a practice sandwich out of muslin. If you like, you can even draw several blocks on it with a ruler and permanent marker. Then experiment with the above to see if you like it before using it on your quilt.