Pretty straightforward, if a bit fiddily. Anyway at least it's an option for Quest users who want to use OpenComposite and have reHUD up and running. Any questions just shout but, I think the video is reasonably self explanatory.
I noticed some people on discord mentioning they were losing mouse focus. This is simply because they are trying to interact with the desktop through reHUD. Easiest way around this is to hold down the meta button and interact with the desktop there. You need to move reHUD out of the way or place the desktop window somewhere other than directly behind the pinned reHUD window.