First you need the extracter, which will extract all the editable .dds files from the packed .wdf files.
And here's the wdfextracter, credits goes to unknownone.
Put all files in CO folder.
Second you need a plugin for photohop that lets you edit and save .dds files.
Third, it's nice to be able to view the files, so here coems a dds viewer in handy.
Lastly, a way to sort out which files correspond to which char etc.