Hi.
1). Very unfortunate choice of the tutorials, the last one especially has a lot of incorrect information.
2). If you want to make maxis match clothing, it would be best to start from frankenmeshing or mesh editing. Making maxis match clothing from scratch is difficult and the tutorials that involve MD won’t teach you how to do that(because that is never their goal);
3). The value being red in MD just means that it is not the same for all selected patterns, not something you need to worry about (unless that is unintentional);
4). I’m not sure about Select all graphics and trims (this is new), but you should uncheck all maps(this is useless for you since you’re going to re-arrange uvs and re-bake textures), « include hidden objects », « include internal shape ». It is best to export as Weld, I guess;
5). Right now the item is very high poly, you want it as low poly as you can make it while preserving important shapes because high polycount negatively affects the game performance;
6). The automatic retopology MD offers fails to do every single thing that retopology should do, so it is pretty useless. Its manual retopology is great, though, so that is something you should consider looking into in the future. For now just modelling in quads will be enough;
7). You do not need to use blender 2.7 and should avoid tutorials that tell you do it, this is a sign they are made by beginners. Everything can be done in 2.79 in your case
( quoted text only);8). The most practical order: transfer weights, uv_1, vertex paint, delete all parts of the body covered by clothing, join the body with the garment, re-arrange uv_0 of your garment, assign a cut number, check how the mesh behaves in-game. if everything is ok, bake textures, split the mesh into meshgroups if needed, make lods. Some of these steps are interchangeable (the first three) but switching others will result in problems or wasted work;
9). You should be aware that in the last tutorial the author says to paint the mesh with 00ff04, which is the wrong hex. It should be 00ff00 and that goes for skintight clothing only, there’s a different shade for skirts and the likes. You can just
transfer vertex paint to not worry about that;
10). You want to bake textures with the body in the scene because it should cast shadows on the garment for some semblance of realism;
11). You see those black triangles after the bake because the uvs of the outer side overlap with the ones of the inner. You should add the inner side only where you can actually see it instead of everywhere, otherwise it does nothing but doubles the polycount. When you add the inner side, move the uvs away from those of the garment;
12). I hope that the author of the last tutorial updated it, but initially she said to never delete the body covered by clothing and its uvs. That is obviously wrong and goes against of how cas game content is made. I cannot tell from the screenshots if you kept it or deleted it, so I’m saying just in case;
13). Bake textures with bigger margins, at least 7:
If your goal is to make maxis match clothing, this is not the right way to get there. In fact, it is best to stay away from these « from scratch » tutorials because in most cases it is clear that, unlike maxis match creators, the authors of those didn’t even look at the existing content within the game before making custom one, so people end up learning things that go against of how cas content should be made to actually function right.