The new Suction Break Fitting that you will receive is the entire assembly. This includes the portion that fits behind the acrylic shell and the clear cap portion that screws in from inside the tub.
To begin, unscrew the clear cap from the spare fitting you have received.
Essentially, all you would need to do is remove the threads from the previous Suction Break cap that broke off inside the fitting. Once this has been removed, you will be able to screw in the replacement cap.
The easiest way to do this would be to use a flat head screwdriver to "nudge" the broken threads until they unscrew from the fitting.
Alternatively, you could firmly extend a pair of needle nose pliers inside the hole and gently turn counterclockwise (to the left) to unscrew the threads.
The fitting itself is glued to the inside of the spa shell. Whichever method you attempt, it would be a good idea to put masking tape around the hole to protect the acrylic.