Hey Dave, welcome. My first attempt with a ladder I screwed a piece of angle iron to the lip of the ladder on one end. Then I drilled several holes in the other side of the angle iron so my screws would screw into the cut, round end, of the log. If there was a bump from a limb or some other reason the angle iron would not line up with the edge of the log I could screw a board to the angle iron and extended it down to the face of the log and screw it on. I attched the angle iron about 2 rungs from the end so I'd have plenty of room to hang the mill on and get started. Then to steady the rest of the ladder I used "pipe hangers", from the plumbing section at the hardware store, they are half moon shaped with 2 holes to put screws in. You can get all different sizes, just match them to the size rungs you have. I have 3 different ladders I use from 10 ft to 42 ft and they all have different size rungs.
A couple days ago my cousin came over to see how the mill worked. I grabbed my milling bag and lost all but 2 of my pipe hangers, and I left my piece of ladder with the angle iron on it up at my farm in WV. So, I grabbed the DeWalt and drilled about 10 holes right through the rungs and used 3 inch deck srews to screw it to the log. It was a straight log and worked just fine. I took some pics and posted them under "White Birch Log II" so you can see it made a good cut. I also took a really boring video and if I can figure out how to post it, it will be there soon. Good luck, Joe.
