Installation of VRIF is relatively straight-forward for most devices. Just start a new project with Unity 2019.4LTS, import the VRIF asset from the Asset store, and you should be good to go. Depending on your device, you may need to install an additional SDK to receive input.
If you're importing VRIF into an existing project, or just want to know how to set VRIF up from scratch, below are some notes for specific platforms.
I recommend using 2019.4LTS as it is stable and has the widest support. However, I also understand the desire to want to use the latest and greatest Maybe there is a new editor feature you can't live without, a performance update you'd like to to test, or you just want to use that lovely new dark theme that's available!
There are a couple of considerations to keep in mind with 2020 :
This largely depends on which devices you need to support or are developing with, as well as which editor version you are using. If you are using 2019.4LTS you can pretty easily switch between the two. However, in Unity 2020 you are locked into using XRManagement. While XRManagement is the path forward for Unity, the lack of built-in support for OpenVR could be a deal-breaker depending on your target devices.