Grab a texture map (you can make one using the free uvmapper classic from uvmapper.com but SnowSultan does better ones by putting in a lot of work). Paint on the texture map as one would paint a pigment texture for the clothing, except that you paint white for where there is still cloth, black for where there is no cloth (e.g. torn away) and grey for thin/worn cloth. In addition you might want to modify any pigment textures so that there's white or "stressed" look around the tears, to make it look less liek the clothing has just ben cut ragged. That should be about the basics for a good start, though there's nothing like experience :)