1.Extract "Realistic Shotguns.esp" in your Fallout4/Data.

2. Open Plugins in ":\Users\--YourName--\AppData\Local\Fallout4" and Add "Realistic Shotguns.esp" after "Fallout4.esm"

Should look like this:

# This file is used by Fallout4 to keep track of your downloaded content.
# Please do not modify this file.
Fallout4.esm
Realistic Shotguns.esp
OtherMod.esp
AnotherMod.esp

3.Put the file plugins in "lecture only" and make Backups in every modification.

4.Enjoy.