You can easily test your masterlist changes before syncing them to the online repository, using the method detailed below, which makes LOOT use a local repository of your choosing than the online one.
- Run LOOT, and open its settings window.
- In the Masterlist Repository URL setting's text box for the game you want to test, enter the path to your local repository. Eg. if you have the Skyrim masterlist repository at
C:\foo\bar\skyrim, then put that in the box.
- Apply the settings, then close and relaunch LOOT. LOOT will now update using your local repository instead of the online one.
Note that LOOT won't keep your local repository synced with the online one, so you'll have to do that yourself.