Hi Eleuter welcome to the site and it's worth while joining the club even though you are in South Africa
I have done a few screen rubbers and it is not a easy job and not easy to explain but I will do my best
it can be done but must not be hurried. Remove the various pieces from the interior around the screens, remove all the Stainless Steel trim prising with a flat blade screwdriver by a twisting motion at each clip don't pull the trim otherwise it will kink
once it's all off it's a case of removing the screws and clips take a few pictures at all stages to help remind yourself how it is all fitted and
which way up the clips are in particular use a Stanley knife to cut as much of the old rubbers away as possible before prising out the screens
don't forget the wires for the rear screen heater! you may be lucky and a gentle tap or two will do the job as you are in a sunny climate I should think all the mastic has shrunk and gone rock hard
Then it's a case of cleaning the openings and the screens, you will need some sealant we use Dum Dum here it is easy to work with and you can use it in your hands. Fit the new rubbers ( Scott's Seals are a reasonable fit) it is far easier if there are two of you to do the job I struggle on my own.
I find spraying the rubber with a WD40 type product helps the rubber into place, ease the inner flap of the seal over the lip of the opening I use an old screwdriver bent 90 degrees about 10mm from the end remove the sharp corners
push the screens into place carefully as you work your way along, when they are in place a few hand thumps all the way around to make sure it is well home
you will find there are lots of spaces to be filled around the rubber and opening it may be worth getting the recommended sealant for your climate ask your local fitter if you have one? apply loads of seal it can be messy but will clean off with a bit of petrol
It's then a case of refitting the clips and screws renewing them if necessary and refitting the SS trim with sealant arround all the clips
my advice is if the trim is hard to clip in place adjust the clips don't bang it it should go back with a firm push of the thumbs
seal arround the roof SS trim refit your interior pieces and with a bit of luck your watertight.
Not an easy thing to explain as I have said Eleuter at least you have a Saloon a lot easier than a coupe to do "GOOD LUCK"