When I click Vertex and then "Sims 2 Unimesh Fix Underweighted Bones V4.09 absolutely nothing happens. When I read manuals, it says that some kind of window or something should pop open. But nothing. I am using Milkshape 1.8.5.
I have learned little bit how to work with joints, but this dress is still clipping from middle. I think maybe I should somehow set bone weights differently (now all selected vertexes that belong to joint show as red color), but I don´t know how to do it. I thought maybe I should try that fix -command, but it is not working. Help please.

FIrst of all, it would help with a bigger picture.

I think the "underweighted bones" tool only fixes underweighted vertices if they're set below 100% to one or more bones. If they already are 100%, I don't think the tool does much, and it won't fix other issues with the bone assignments.

If one of the issues is that the legs are poking through - does the dress need legs, as in is it meant to be alpha-editable? If not, and the dress is closed at the bottom (only shoes visible), you can remove the legs but keep as much of the shoes that show. It would help with the clipping.

I am so unbelievably frustrated. When I open the mesh in Milkshape, and select a joint from the "joints" section, for example, r_calf, I see that multiple vertices are assigned to this bone Or is it a joint? Some of the vertices are yellow, some are green, and some are orange. Apparently, the color of a vertex somehow defines its weight in the animation. Well, then I select a few vertices using the selection tool and click "assign," and these selected vertices are added to the r_calf joint or bone. But every time I click assign, the vertices turn bright red. Even though I try to use the gradient slider, I can't get them to turn green or yellow. I just don’t understand how to do it. And I've been Googling like crazy for hours, but I can't find a clear, simple guide on the topic.
I even tried to use help of chat gpt, but even still I dont get it.

(Bone/joint has pretty much the same meaning in this context and are in most cases used interchangeably, so you'll see both used in tutorials and such)

Also no idea how ChatGPT can be of help in this. Would rely more on tutorials by people who know their stuff.

You kind of need to understand what you're doing if you're changing the joint weights manually. They're attached to 1-4 joints, depending on placement. In some areas more refined weights are necessary, while in other areas there's just a split between two joints, so those need to be divided (50/50, or 25/50, or whatever works). This is so the clothing gets a smoother transition between the joints.

Looking at how similar meshes are weighted is a good idea. You can select a vertex, choose "show" (which shows you all the weights). You can change the weights and then click "assign", or you can copy the ones you had to a new area by clicking "show" then selecting the vertices you want to assign and clicking "assign". Make sure they always equal 100%

The "Draw vertices with bone colors" tick box is useful - the mesh should roughly keep the original color outlay. If you see anything that suddenly changes color, there's a bug somewhere (a left joint selected instead of right one, or similar). Personally, I tend to use this one a lot, but I turn it on and off, since it's difficult to see selected vertices when it's on.

"Draw vertex weights" shows the weighting of a particular selected joint (White 0% --> green 1% --> yellow 50%--> red 100% weight) - it's more to show the flow of that single joint. You don't need to have this setting on at all times (can be confusing).