Insane bug in game - exploded leg

Hi. For the future, please always provide the files.
From the pictures I can point out 6 mistakes:
1). See how the mesh distorts where the skirt is meant to be? These are most likely skirt bones. They affect your mesh but not the item you cloned to start a package;
2). Notice that I said « where the skirt is meant to be » instead of « where the skirt is ». You can see that not only the skirt distorts but the leg as well. This happens for two reasons:
a). You joined the dress with the body and then transferred all the data from the reference. The body’s data was overwritten and now a portion of the leg is rigged in a way a skirt on longer dresses is;
b). You chose a reference that was much longer than your dress. Items of different shapes are adapted in different ways and you should always choose the reference that is closest to your garment. Mini dresses are not affected by skirt bones at all;
3). Did you notice that your uvs look stretched vertically? TS4 CAS uv space is rectangular while the default uv space of any program is square. When you assign the rectangular template to uvs generated in the square uv space of MD, they stretch. You must shrink them to the original size. Also, you should try to make clothing in a similar way it is made in real world (front, back, right sleeve, left sleeve) instead of just front and back patterns. The bigger the pattern, the harder it is to fit it in the available uv space, the smaller you will have to make it, and the worse the texture quality will be;
4). The most complex maxis meshes almost never go past 10k tris. Yours is high poly enough to crash the game for most people. Fine details like wrinkles should be preserved through textures, not mesh. You can bake the textures from a high poly version to a low poly one. Before you make a high poly version, smooth the avatar (see how jagged the mesh looks?)

I would advice you to watch a different tutorial because the one you watched was not good.