First... download the mod! Click here to access the download page, then find the relevant patreon post, scroll down to the bottom, see what it says about each download mirror, choose the one you think will work best, and download!
Next, open the .RAR you downloaded with a program like WinRAR (download here), then you want to extract the whole "FIFER's FIFA 21 Realism Mod" folder somewhere onto your PC. It's best to put it in a specific mods folder that you have, like Documents/FIFA 21 Mods. Never install it in any sensitive areas, like Program Files or System32!
After that, install the FIFA 21 Mod Manager. Note, this is different from Frosty for FIFA 20. You need to have the latest version at all times, which you can get here. Then, extract the whole folder somewhere on your PC, same steps as above. Again, don't install it in Program Files or System32.
Now, navigate to where you installed the FIFA Mod Manager, and open the EXE. Once it loads, it should automatically detect your FIFA 21.exe, if it doesn't, it will let you choose it manually. Then, use the "Import Mod(s)" function of the tool in the bottom left hand corner. In the window it opens, find the place where you extracted the Realism Mod, and import whichever mods you want to use, which are normally the main 13 mods, and then some other addons. Once you have selected and imported those, wait for them to install, then select them all (or as many as you want), and click "Apply Mod(s)" at the bottom. Make sure to read the mods carefully to know what order the mods should be in. Finally, on the Applied List, click the Locale dropdown and choose which locale.ini you want to use.
Finally, load up FIFA. To load the squad file, go to the Realism Mod folder, and find the squad file you want to install. If you are using the national teams mod, go to the expanded national team experience folder, and find which squad file you want to install there. If not, the one in the main realism mod folder works. Then put it in Documents/FIFA 21/settings, go to the Customize tab in game, then Profile, then Load Squads, and select the squad file.