This is not a job anyone does on a regular basis, so most of the suggestions,so for, are a bit common sense and practical experience rationale. There is likely several approaches you will have to try, before you get one that works reasonably well. I don't think any approach will make this particular job easy and/or smooth sailing.
If all you have is an old ladder, that approach, to me, will make any cutting approach more tedious, difficult and likely not too safe, depending on your scenario. A scaffold rents for $10 a week, here. A scaffold will make any cutting approach, no matter which, much more convenient, easy and safe.... and the end result would almost certainly be more satisfying.
Sonny