the process of 2's complement really doesn't know anything about positive or negative. it just means "flip the bits and add 1".
How those bits are interpreted are up to you (and the computer). If you're really asking "how do i find the bit pattern of a negative integer", or "what integer is represented by such-and-such a bit pattern" that's a little different.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors