Start with the answer to something similar you've seen before.
Use common sense (?) to figure out which aspects of a formula aren't relevant.
For example, in T(n) = 2 T(n/2 + 17) + n, the 17 is probably not that important since n/2 + 17 is a lot like n/2 for large n.
The book lists a number of subtleties that crop up and heuristics to help get you out of trouble.
