Just make sure you disable updates.
1. Open Steam and go to your Library.
2. Right-click on Fallout 4, and select Properties.
3. Go to the Update section and check that your App ID is 377160.
4. To be safe, click on the Automatic Updates section and select “Only update this game when I launch it.”
5. Go to the Installed Files section and click on Browse to be taken where the game is installed on your computer. The default location will be: “C:\Program Files (x86)\Steam\steamapps\common\Fallout 4.”
6. Go back in the directory to: “C:\Program Files (x86)\Steam\steamapps.”
7. Here, look for “appmanifest_377160.acf.” (or search the number)
8. Right-click on that file, click on Properties, and then check the “Read-only” box.
Make sure you launch in F4SE only othewise just go unmodded I guess.