Creasing textures

Well im having trouble understanding how to crease edges to put skins on , i dont get how works like if had a robe then pants under it how that world work or were put it and sometimes when i have selected they just make the face a box that fills the texture, idk why , please some tutorials detailed on how to do the creasing and how it works and the concept of how works and exact how do it would be helpfull.