So, I picked up a hair but the ponytail is kind of weird, it's modeled so that it's like defying gravity and its pretty long, so I opened the package up in TSRW, exported the mesh as an obj. First I imported it into Milkshape (1.8.4) because I know that I can export the obj and TSRW will accept it without issue, but I've been using Blender (3.4) for so long that I kind of forgot how terrible Milkshape is. Maybe I just can't find the options, but the selection tool is too basic for fixing this. Simple box select without being able to add or subtract from the selection isn't going to cut it with this mesh.
So I imported it into Blender. Much easier to fix there. I L selected all the parts of the ponytail with ease, rotated it and made it shorter then exported it as an obj. But, when I try to import the mesh in TSRW I get an error, "index was outside the bounds of the array". I didn't change the vertex count at all or rename anything. I don't know why the obj can go from TSRW to Milkshape back to TSRW without any problem but not from TSRW to Blender back to TSRW.
So, how do I get this to work?

TSRW often doesn't like to import as object, especially if something you did increases vertices count by even 1.. You'll have to convert object to geom to .wso using MTK. Of course you can convert object to .wso with MS, but I'm not sure how that works carrying over bones.

The vertices didn't increase at all.
And you're going to need to be more detailed in those instructions. I don't know what those acronyms mean or how to convert to geom or wso. Blender can't import or export either of those formats.

MTK is Meshing Tool Kit, and available here on MTS, I believe under tools and utilities in downloads section. Amazed you don't have it, as I can't do anything without it. Also, Smug Tomato's geom tools for Blender are in the same section.

is there some sort of video tutorial for using them?

I never saw any for MTK. It's been around forever. Open tabs and look. Pretty self explanatory what each tool does. I can help with specific questions. As for Geom tools, Blender manual should show how to add tools, and there's a bit of explanation on Smug's dl page.

I know how to add plugins to Blender, but I need a video for actually using all these tools. I have searched. I've looked at several written tutorials at this point and they haven't been helpful. There aren't enough visuals and it seems like important details are missing or not explained. I end up with more questions than answers.

MTK has some pretty basic stuff, and some more advanced. It's an old tool, so probably no videos. What you need from it very basic and simple. I say that only because I figured it out for the most part. ))) Some stuff, I have no clue about still!

If you have the hair as object, open MTK and click the tab Conversion Tools. Do NOT go directly to OBJ to WSO!!! Blender objs don't have bone assignments. What I do with most meshes, is convert OBJ to GEOM 1st. If you ever do garments, MTK will split UV seams and sharp edges automatically for you. For hair, it generally won't matter, unless one of those TS4 hairs, or from a foreign model sometimes.

You will need to open your original package in s3pe, and export the L0 geom. In MTK, open the Auto tools for GEOM tab. On the left, is assign bones. Open it and import your mesh (geom to modify) and the geom you pulled from the package as the reference. Click. Now back to conversions tab, and on the left, geom to TSRW wso.

If you have morphs, export the original as wso from TSRW to use as reference. Most hairs don't have morphs, and hi poly hair will usually explode if you try to add morphs. But, you can see those tabs for assign bones, seam fix and morphs. There is also a seam fix in the auto tools for geoms. You only need to use one or the other for garments. As long as you do not alter position of vertices at neck, waist or ankles, seam fix will automatically match your mesh's bones and normals to standard EA.

FYI: MTK has a name those files tool, so if you can't find L0, 2 options are export them all and the biggest one will be L0, or use name files, call things whatever you want, then when you open the package in s3pe, you'll see everything labbeled, except txtc's and maybe something else. Good luck!

The OBJ to WSO part of MTK says it also adds bones, the bones from the wso reference - and that's indeed the case IME, just FYI.