I'm writing fairly detailed here just to be sure.
The filter comes in a zip file called "assettocorsa.rar". Unpack it. You will have a folder called assettocorsa (not to be confused with your game install folder). Below it is a system folder. Copy that folder to your actual assetto corsa game folder.
Confused yet?
Example: Let's say you unpack the assettocorsa.rar (the mod) to c:\mods\
Let's say your game install folder is c:\games\assettocorsa
Now copy
c:\mods\assettocorsa\system folder to
c:\games\assettocorsa
It should not ovewrite anything in your game folder's system folder.
Then start up CM. Go to
Settings -> Assetto Corsa -> Video. Under
Post-Processing select "RTFX 2020 ACC" in the
Filter dropdown.
Then go to
Drive and select one of the SOL weather types. Now start the game.
Open the
Sol Config app in-game. Increase
blacklevel compensation (value depends on yout monitor). On my the default (0) is way too dark. Then set
colors whitebalance to 3. Again, this depends on your taste. 0 and below is blue-ish. Above 0 is warmer.
Then go to page 1 (at the bottom of the
Sol Config app) and disable
ppfilter load basic custom config (although I'm not quite sure if this is necessary). Then just play around with
ppfilter brightness and
ppfilter contrast until you are happy. I set them to 110 and 95 respectively. Overcast was a bit too dark on my monitor by default. 110 makes it look a bit more like a bright day.