1. Navigate to the Master Page Gallery (Site Settings -> under Web Designer Galleries -> Master Pages).
2. Select Files tab and then Upload Document.
There are three possible options
Target Scope – URL of the website this override applies to. This can be a relative path to your Site Collection or subsite.
For example, 107 would be the Target List Template ID for a Tasks List.
Once your done adding document properties Save it.
Original article : Here