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