Take your time and pull the whole thing apart. The Haynes manual is sort of Ok, but...
Front seat bolts can be accessed from underneath where the exposed thread rusts and gets damaged by stones - A wire wheel on an angle grinder usually cleans these (and seat belt bolts) nicely. Don't be afraid to grind damaged ends off and replace the bolts. The inboard seat bolts may be holding the exhaust heat shield on, and the lock nuts will make these impossible to unscrew from the inside, (505 pugs are the same) so it's important to get underneath for a look before you try to pull the seats out. (See sticky on working under Citroens) The underlay is sisal or similar, it will be rotten by now as well as mouldy, so remove it all, re-paint the floors and replace with new. The carpets are synthetic. Be careful getting them out as they have a thick moulded foam section underneath that will tear and fall off if it's full of water. Getting the front carpets out may require two people. Impact screwdriver is recommended for the tread plate screws. It's probably a good idea to pull the sill trims off at the same time as there may be rust holes underneath.
Good idea to pull the grille out and the anti theft shield on the bonnet catch as the bonnet release lever may have to come out to get the carpet out. That way you can still get the bonnet open.
I have had to drill the head off a seat mounting bolt, drill through the rest oversize after the seat was out and replace with a bolt and nyloc nut the next size up, expect it to fight back. Get penetrating oil on everything for at least a week. Little bits of rag help to soak various bolts.
The heater knobs pull off, best done when it's nice and warm inside the car. There are all sorts of bit, such as heater box flap seals, which should be replaced. The heater core is always hot in a CX and leaking flap seals mean weak to non-existent AC. The heater assembly looks like it's the size of a small bus, but isn't hard to work on once you get it out. It has to be pulled backwards into the cabin. The heater fan in a CX can be quite weak, so worth getting it apart, re facing commutator and new brushes, because of the difficulty of fixing it later. A more powerful fan is recommended.
Some cars had rear air added. The hoses run under the car inside the left subframe rail. These get porus in their old age and let the gas out.
If the air intake stack drain is blocked, pull the whole thing out (allow half a day) and you'll find all sorts of muck between the cross member and the bulkhead. While you're in there- There's a not very convincing seal between the intake stack and the cabin fan. If it's not sealing it lets hot oil smells in. Some cars had a re circulation system added (in the engine bay above the steering wheel/ below the coolant tank) which can also leak both oil fumes and water... There are also the bits of cloth ribbon which are supposed to hold the handbrake cables in the right place. Easy to get the cables out with the heater out. It will be really quite dirty down there.
Check the rubber Tees in the return hydraulics. There's one near the steering which can be a real pain when it lets go. Replace suspect ones with brass Tees. Be prepared to replace various bits of rubber return hose. There's also the rubber steering coupling down under the intake stack. The procedure for replacing it is quite involved though. The Haynes manual covers it.
You will need quite a bit of storage space, and to take photos and videos and make diagrams.
If you catch up with all the age and wear related nasties, the CX is a really nice car.