First I locate the error in a small block of code. Then I think of all possibilities that could occur making my application crash.

If I can't solve it, then I start checking all variables (pointer, numbers, more pointers, strings, again pointers) which have a relation to the false code on weird behaviour. If I detect abnormalities here, I usually find the bug soon as well.

No abnormalities? Then it's time to have a little private conversation with the piece of code. I make a copy of the whole script and cut as much unnecessary code out as possible, or replace with dummy data. Usually it will work then, and from here I try every possibility to make it crash the same way it did in the old code until I found out why.

If it does crash right after placing it within it's new scope, I start to get hopeless and post on the forum asking why my game hates me and break down in tears.