Download vMix onto your computer, that can be downloaded here
Configuring Output Settings
This is done by clicking on the cog icon next to External and selecting External Output Settings
This will launch a window that will default to the External tab. This will be your main program output where you will select which channel on the Blackmagic Decklink Quad 2 card will be your output.
Make sure the check box External Renderer is ticked.
Select the device you want to use. In our case this will be the Decklink Quad and you will want SDI 1 so the selection will be DeckLink Quad (1) 1.
The Alpha Channel can be set to None, Premultiplied or Straight. In this case we will choose None.
In this example we are only testing out 1 Fill and 1 Key so you will need to navigate to External 2 tab to disable it.
Make sure that the External Renderer box is unchecked and all the other fields are grayed out to show that External 2 have been disabled.
Now add your Production Link into vMix. A Help Center article on how to do that can be found here.
Once you have LIGR into vMix, hit the External button, that will turn red and you will see an SDI output on your monitor.