It's quite ez to use ^^ :
1) go in assets0.pk3, and take the folder "_humanoid" located in models/players/
extract it to gamedata/base/models/players/ folder
2) Download modview, and etract the modview.exe to your desktop. Now launch it.
now, go on file/open, here get the path to your model.glm of your skin, then open it.
**** TO WORK, THE _HUMANOID FOLDER MUST BE IN THE SAME FOLDER AS YOUR SKIN FOLDER !!! ****
In the tool : to change point of view : mouse left click, then drag.
to change fov : mouse right click, and drag.
each time you edit a texture, save it, then refresh your model : it's the third icon starting from right at the top.