While waiting for Steam to download the majority of my Fallout 4 “retail copy”, I did this. The tricky part was to figure out how to activate a plugin, but others figured that out for me.
This mod contains two plugins: one doubles the XP rewards for quests and the other halves it.
To install, copy ONE OF the plugins to the data folder.
To activate, append the file name of the choosen plugin into the plugins.ini file.
Find your Plugins.ini file at c:\Users\(YOUR_USER_NAME_HERE)\AppData\Local\Fallout4\
The launcher will keep overwriting the file, so you need to set it to read only.