right click on the .wmz file, choose rename, change the last three characters to 'zip' (no quotes), hit enter. Then double click on it and your unzipping proggie should appear. Extract all the files to a folder. You must maintain the folder structure of the skin or it won't work when you put it all back together. Make the modifications to the graphics, making sure to keep the names the same. Zip the whole thing back up. Change the extension back to .wmz and you should be good to go.
Oh, and btw, WMP uses color maps (at least it did when I was skinning it) for identifying different parts of the skin for mouseovers, activation, etc. They should be easy to recognize. Don't change any of those colors.
Hope that made sense

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