Okay, so I never found a good tutorial for this, so my way to learn how to do this is trying, asking, trying, trying, asking and trying.

The thing is I'm trying to make backscars as a "glove accessory". I've made a "testing scar dds" which doesn't apparently look very good, but it's a test and the most important thing is that it doesn't work.
In the Cas Texture Unitool preview, everything looks as good as it should be:



In the game.... well, no.



That happened when I did this: I used the male top as a base for my .dds-base texture. I replaced the top's base texture with my own and used an RGB mask too. I classified my new cas part to be "Accessory - Glove".
When I saw that happen I thought the problem must be the top's mesh, so I tried those same things not with the male top, but with the nude version, so there should not be any mesh. whoa, there IS!

Well, this happened;



I'm a novice and can't really figure out what's wrong. I don't think I've messed up something with the .dds-file, I've done facial scars before and they all worked fine.

Any ideas?

It looks like it's either overlapping something, or the whole creation got corrupted. You got the glove accessory thing right, but you might have used the wrong 'map' for it. As you might know, there is a Multiply map, a specular map and a mask map. You have like 5 others, but these 3 are the most important ones, especially when you're making something like this. For textures on a skin, you always need a Alpha channel. You can add one in the channel tab in Photoshop. I guess Gimp has one as well. Just create a new channel like you would make a new layer. The new channel will be called 'alpha' by default. Go back to the RGB and select your scars with the magic tool or the quick selection. Go back to the alpha channel and draw the selections white. Delete your sim skin base (which it seems you did) and export it as a DXT5 (the first one you see). Then, you need to load it into your Multiply map. If you don't know how to delete the soecular or the mask in CTU, you could always open up your exported package into S3PE and delete a black looking image by using right click (or at least one with a black background). Some red coloured image (maybe it has any yellow and purple in it as well).
I guess this could be a bit confusing, so let me know if you have any problems. Else you could always try different ways of doing it. There are like 2 other ones I can think of, and these are either TSRW or using a package that is made the same way like you do and replace the image within with yours, but I guess that could break most CC creators copyrights.

I think I tried that exact same thing, but it didn't work.

Thanks anyway. I sorta figured it out; I tried to not recategorize any top as a glove, I imported my files straight onto a glove and that's it.
The only negative thing is that the 3D preview doesn't work with a glove, 'cause it has no mesh to load, but I can always take a look at it by viewing it with a top mesh and then change it to a glove.

In CTU there is an option where you could just reload the texture, It's next to reload mesh in the 3D view Although I never used CTU for any texturing work so, No idea if that will work, but it's worth trying ^-^