I did both front sections too, they have a little bit of variation on the edges since you don't put it over what little inner arch intrudes into the cabin and for the driver's side there's a big round plastic stop on the back of the carpet to avoid. With that all ready to go in the car I found it easier to remove the centre console to install it. I also wanted to clean the centre console up since I wouldn't easily get another chance to do that and it's a much easier thing to clean out of the car. To remove the centre console there are very few fixings: one big plastic screw at the front, the gearknob, a small bolt under the plastic grommet, and a small nut hiding behind the blanking pocket (that someone has used as an ashtray in the past, which is a bit annoying). If you ever wondered why the bottom of the handbrake slot on the centre console had that bigger wide bit at the bottom, that's so you can get it over the handbrake when you remove and refit since the handle is too wide to go through the other sections easily. The rear pocket prises out, there's two spring-tabs at the top and and at the bottom.
This is the bolt the big plastic screw at the front goes on. You can unscrew this with a regular screwdriver, you don't need a massive one, it's not very tight.
This little bolt is hiding under a rubbery cap that simply prises off. The front portion of the centre console overlaps the rear. You can remove the rear portion without removing the front, if you're careful, but you do have to undo this bolt to do it.
The gearknob simply unscrews. There's a spring underneath which isn't under any tension, and the black plastic portion of the gear lever goes some way underneath the boot and onto the metal part of the gear lever, so be aware of that when installing and removing.
The nut hiding behind the pocket at the back of the console also has a washer, this is the prevent the plastic cracking from the nut being over tightened. Easy to remove.
With all that done, the console is very easy to remove from the car and very easy to refit. One of the easiest centre consoles to do since there's no sneaky hidden fastenings or really any particularly twisty way to get the thing in and out. With that out and cleaned, I could reinstall the carpet which was equally very easy. I didn't remove the rear door trims, I found I could just push the carpet underneath them when the screws were loosened a bit. There's four fir tree style trim clips under the rear bench to hold the carpet down there, again you needn't remove the seat, you can push the carpet through the gap between the floor and the seat if you lift the seat base up a bit, then fold the seat base fully forward to push the trim clips home. There's two screws holding down the plastic trims at either end of the seat bench that also help hold everything flat.
For the seats, it's 5 allen headed bolts. I found it easiest to push the seat all the way back to do the front ones, then all the way forwards to do the back ones. Don't fully tighten the front bolts until you've got the rears in, the floating captive nuts Citroen use for the seat bolts give you a lot of adjustment that should make fitting the seats easier. Unfortunately, I didn't have a socket small enough for these, so had to do it with allen keys, which is a chore. The front ones aren't very visible and removing the seat base doesn't make life much easier.
At the rear of the seat the sill side is a single bolt, the seatbelt side is two bolts and a metal plate. This metal plate serves in part to cover the raw carpet edges but I suspect is also to help spread the load for the seatbelt, there being two bolts here rather than one is probably also to do with that, I expect.
So, with all that in I vacuumed the carpet to get the dust from my shoes out and the fibrous bits from the underlay off and enjoyed the much softer feeling carpets. I swapped the front seat bases too, since there's rarely anyone in the passenger side, I thought it best to put the original fabric base on that side to reduce wear until I can make up my mind what I'm doing about the faded and worn seat fabric.
I reinstalled the wind deflectors too. On rainy days they make the car so much nicer to be in since I can crack a window open and have some fresh air coming in without getting wet. It's so much nicer inside for having the new underlay and having the carpets shampooed, it's a relief not to have to put up with the boom from having no carpets or passenger seat in the car.
