Location of the bottle
The manufacturer instructions say that the bottle should be within the rollcage but the FIA Art 253 7.3 section also says that the bottle might be in the trunk as long as it's 300mm for outer body panels. Note that the bottle must be mounted transversally so that in case of a frontal impact it doesn't fly into the car. Anti-torpedo tabs are required.
While the ARA rule only specifies 'Activation point for the fire system must be located within easy reach of the driver and co-driver when seated.', do not overlook the manufacturer instructions that might also require the external release cable to be installed as indicated below. If you are using the Lifeline system like the one below, you MUST install the external release.
A common mistake when installing the release cables is to install them backwards as the moving handle is usually the opposite of a handheld fire extinguisher. Make sure you locate the cable guides (usually gold in color) and route the cable through that side (see picture below).
FIA Article 253 section 7.2.3 specifies that the outside trigger must be combined with the circuit-breaker switch. They further explain 'For WRC type cars, the triggering of an external or internal extinguisher must compulsorily bring about engine and battery cut-off.'
Since this combination of the fire trigger with the circuit-breaker is not in the manufacturer installation instruction, ARA will probably not enforce that rule until further clarified but it is probably a good option to locate your external trigger close to your external master disconnect switch if you have one. If you use a system with an electric release, it would probably be a good idea to wire it with your master disconnect if you are using a system such as a Cartek GT.
The recommendations vary but usually 4 nozzles are installed under the dash pointing toward the driver and codriver legs/seats and 4 in the engine compartment pointing to the heat and combustible sources (exhaust, fuel rails, etc...). You cannot just hook up the nozzles to the hoses and let them hang as they might move when the system is pressurized. You have to use brackets to hold the nozzles in place (see install video below).
Servicing the bottle
To send the bottle back for recertification, you need to disconnect the release cables, straps and push the collets on the T-fittings to release (using a 13mm wrench to push on the collet helps) and pull out the tubing. There is a demo of how to pull the tubing out in the Novec install video above.
Note that AFFF systems can freeze in winter and you might have to remove the bottle to keep in indoors during the colder months. The water will freeze at 0F but the bottle can sustain damage with temperatures under 15F.