As I said in my last post in this thread (in 2019) I did manage to figure it out, but I never did find a good tutorial anywhere.
The wiki page here has the best information available that I have found:
http://wiki.tesnexus...cter_animations
From here, unfortunately, it's a lot of experimentation to figure out what works and what doesn't. There are a lot of hidden gotchas that aren't documented anywhere. For example, the GECK seems to store hidden info about the animation that you cannot edit. It reads this info in from the animation when you assign the kf in the GECK. So if you have something wrong in the animation, like for example you are using Blender's default looping tags instead of the ones that the GECK wants, that info is hidden in the GECK so even if you fix it in the kf it won't fix it in your mod. What I typically do to get around this is I keep changing the name of my kf so that the GECK constantly thinks I am assigning a different kf, which forces it to read in whatever this hidden info is each time I change the animation. So for example I'll make MyAnimation1.kf then MyAnimation2.kf etc until I get the animation working the way I want, then I change it to the final name that I want like MyAnimation.kf.
Other than that, my only advice is to just muddle through it and figure out what works and doesn't work for you.
If you manage to find any good documentation or tutorials, let us know, because I haven't found any and I don't have the time to document what I have figured out.