Help edit skeleton.nif

[Deleted47683178User]

So, XP32 is a great skeleton for all possible uses. But well, I was practicing some modding and found myself stuck at editing skeleton.nif files. I wanted to try connecting a tail to a normal skeleton, basically, create XP32 but without wings and stuff. However, there are absolutely no tutorials or explanations of how these files work and what they mean. I tried to change my skeleton.nif in NifSkope, but I got some broken terrible looking waist and I couldn't add a thing to it because I'm basically shooting while blinded. The only useful tutorial I found was about some weapon stuff, but it also didn't help me much.

Where can I find info about these files? In what program should I edit them? Are there any proper tutorials explaining this stuff, or is it something absolutely obvious and it's just me not understanding it?
P.s. I would just like to take XP32's tail bones into my skeleton, I wouldn't mind to cheat it this way as it's just practice and I'm not going to share it anyway. However, if I can understand the idea I wouldn't mind remaking the tail from scratch.
P.p.s. I want HDT Tails Wearable (https://www.nexusmods.com/skyrim/mods/58165) to work with it. Basically, it's the only reason I'm doing the entire thing