Build that up from the inside out:

1.    12 
2.   -12 
3.  |-12|
4. -|-12|

That is, we start with 

1.    12
then put on its sign:
2.   -12
then put the absolute value bars around it:
3.  |-12|
Finally put the outside negative sign on it:
4. -|-12|


That is, start with the positive number
1.   12
So far, the value is 12.

Then put on its sign:

2.  -12
So far, the value is -12.

Then put on the absolute value bars

3.  |-12|
Absolute value bars around a signed
number mean to make the sign of the
number positive and erase the absolute
value bars. 
So far we have 12.

4. Then finally put on the outside negative sign.
   -|-12|

That is, we put a negative sign onto 12,
getting final answer -12.

Edwin