I tackled this project this weekend, and it went much smoother than I expected, at least as far as I can tell now. I did take some pics throughout to share:
This first one is where we started:
Step One was to mark on the liner where I wanted the shelf break to be. My water level was down a bit, and was actually right where I wanted my shelf level:
Clearing away the mulch and dirt to get access to the liner. I had forgotten that I left the old liner in place as an extra layer of protection:
From there, I cut in with the shovel for about an 8" shelf, that should be around 4" deep. I got in a hurry and forgot to take a pic before placing the lining back down. I was able to peel it back a bit just to show:
Finally, a final pic once the water had settled back down, and I had filled back in. I kicked a ton of muck in while I was working, so it took a while for the filter to do it's job. It's looking pretty good again, and just needs some clean up. I placed those two rocks to keep the liner down while I was filling.