The most complete method that we've come up with for creating our DR documentation, involves going through a full cycle (or two) of installation, and documenting each step along the way.
I realize this can be a bit difficult if you don't have a test (or replacement) system to use to create your documentation - but it's worth lobbying for running through this cycle at least once.
(I recommend twice, the second being done by someone not involved with the project - this is how you test the documentation for future admins, who may not be as experienced with the process.)
A side effect of the above is that your documentation grows fairly large - last I had to do it, I believe the completed installation manual for our database servers was 30+ pages.