After you have PurrNet in your project, simply create a new Object in your scene and call it something like "Network" or "Network Manager".
To this object, you first add the "Network Manager" component of PurrNet.
Populate the Network Rules field (Recommended: Unsafe) This will cause the Network Manager to fully unfold.
Populate the Network Prefabs field (Recommended: Just click the "New" button)
And that's it! Try it now! Start your Unity editor and watch the network manager light up with both the server and client. This means that you are now running as Host.