It's taken a lot longer than I thought.
Partly, this was due to finding new tricks/plugins that improved the process. Partly, it was due to finding unexpected bugs.
However, the main delay came from me not just wanting to do a "do this, then this' tutorial where you come away with no context whatsoever about what's actually happening. I want people who watch the video to get a good idea of what AVISynth is, and what FFMPEG brings to the process.
So, I'd start recording a tutorial, then get caught up in a tangent. An hour and a half later, I still haven't finished the video.
Plus, I make mistakes. There's a lot of material to cover, and I found myself leaving out crucial points (or ones that clear up confusing areas). I could have tried writing out a script, or just patching over my mistakes. However, it turned out that just repeating the actions so many times that you can practically do them in your sleep eventually got me to the right place. I still had some audio issues to address in post, but my overall presentation quality is much improved from earlier attempts.
I could try adding more production values than just a screencast. But at a certain point, you just have to release a project. So, here without further ado is my first full-length tutorial. Because the total video is around 45 minutes, I will probably add chapter links on the video at a later point. For now, though, here is my work. Constructive criticism is welcome.
UPDATE: The AVISynth wiki has been updated in the last couple of weeks with new download links for many of the filters, and the plugin pack is no longer in a place I can easily link to. I've updated the instructions below to reflect this, and made an update video to the tutorial (linked below), but the instructions for setting up QTGMC in the above video are now outdated. I will try to keep this page updated if other significant changes occur in the future. Anyways...
Here's a written description of the process, including links: