The method for this is very simple. Remembering it, however, takes some work, so practice this as much as possible.
Here's an exam question:
"Represent the denary value -18 as an 8-bit two's complement binary integer"
To convert a negative denary number to binary using two's compliment, we first have to (ignoring the fact that it's negative at this point) convert it's denary value into binary.
I'm using 5…