Skaruts requested that the Wiki entry on skins is not that easy to understad, so I tried to update it. This would be the (unformatted) updated version. I am thinking about adding a specific example later on, but for now it is not included. I would appreciate any constructive criticism before I actually replace the olde version!
The skins functionality in D3 allows a single model to have a number of textures ("skins") which can be selected from a dialog in the editor. This provides ease-of-use for mappers and (possibly) reduced memory usage as the model only needs to be loaded once.
A skin file has the following basic structure:
model [complete path of the model]
[material path of texture to be replaced] [complete path of replacement texture]
The title has the format
Your skin name has to be unique and like all definitions in TDM should not contain any hyphens (-).
In the inner part of the brackets, you first define for which model the skin can be applied. If you do not define this, the skin will not show in the model menu in DR. At this place you can define several models at once, if they share any textures you want to replace. Make sure to use the complete path of the model. The easiest way to do this (and to avoid any spelling errors) is by inserting the model into DR and copying the "path" spawnarg to your skin file.
In order to replace textures, you first need to know which textures are present in a given model. One easy way to do that is to open the model preview window in DR. On the left side, right beneath the models list, there is a list of all materials used in this model. To replace a texture, you insert the whole path of the texture you want to replace and, seperated with a space (or for better readability a tab), directly write the path of the material you want as a replacement. Each texture that does not get listed here, is not replaced. If you want any part of the model to not appear at all you can use the nodraw material ("textures/common/nodraw").
If the model should appear completely black in game, this means that TDM cannot find your definition for the skin. Check for spelling errors! Similarly, if any part of your model appears black, the defined texture cannot be found. Keep in mind, that any custom texture you want to use has to have a material definition!