1. Download the mod from Paradox Plaza.
2. Open %userprofile%\Paradox Interactive\Crusader Kings III\mod\pdx_39020.
3. Extract the .zip file to a folder.
4. Open the folder.
5. Copy the Realms in Exile v2.3 subfolder.
6. Paste it to %userprofile%\Paradox Interactive\Crusader Kings III\mod.
7. Open %userprofile%\Paradox Interactive\Crusader Kings III\mod\Realms in Exile v2.3.
8. Locate the descriptor.mod file.
9. Open it using a text editor (such as Notepad).
10. Change the line starting with path= to path="C:/Users/USER/Documents/Paradox Interactive/Crusader Kings III/mod/Realms in Exile v2.3". Make sure to use forward slashes and not backslashes. You need to replace USER with your user profile folder name.
11. Copy the descriptor.mod file.
12. Paste it to %userprofile%\Paradox Interactive\Crusader Kings III\mod.
13. Rename it from descriptor.mod to Realms in Exile v2.3.mod.
14. Open the launcher.
15. Remove the mod downloaded from Paradox Plaza and enable the one you just added.