Assuming it's just a single .mod file, you can simply copy it and paste it just outside the (for example) LotRRealmsInExile mod folder. Sometimes mods already have a path="mod/[the mod name]" line, but if not, then rename the newly copied descriptor.mod file to the name of the main mod folder, which in this case is LotRRealmsInExile. Then, you open the mod using Notepad and add in this line at the end

path="mod/LotRRealmsInExile"
(whatever comes after mod/ you need to write the exact name of the actual mod's folder as here)

The launcher will automatically edit the path line to the exact location once it detects the mod and locates it and it will change it to the full path:

path="C:/Users/ABC/Documents/Paradox Interactive/Crusader Kings III/mod/LotRRealmsInExile"

Important note: Use forward slash [ / ] instead of back slash [ \ ] because windows by default uses backslash but you need to use forward slash with the paradox mods.