How does someone decide from what direction to approach the twelve-Nidānas?
Because I believe I understand twelve nidanas, I suggest you approach them from the bottom up.

This way, each next nidana up will be the "cause" (or, more precisely, the ground or the context) for the one below it.

As I said here, I believe the relationship between nidanas is that of implication. Be sure to check out the thoughts of Nanavira Thera and ven. Buddhadasa for useful hints. I do not agree with all of their conclusions but they are on the right path, as far as I can see.