How To question
 

I have a GL Fender jazz, want a bit more fatness, so I bought a set of DeMarzio DP123's.

Is the replacement process essentially a 1 for 1, or should I do anything differently? This is my first time with this.

The DP123 is a humbucker.
So you need to solder the two "inner wires" and isolate them.

For DiMarzio this means:
Solder the green DP123 wire where the black wire is now (to housing / GND).
Solder the red DP123 wire where the white (or yellow) wire is now (HOT).
Solder the white and black DP123 wire together and isolate them (with tape).

That's all ...

Thank you sir!