Mods FAQ: Links to Tutorials & Resources

Hi! I'd hate to bother you but could you by any chance link me to a tutorial or something of the sort on how to edit skins? I'm currently trying to edit skins I've downloaded for my sims so I can tweak a few things I don't like. I've converted them from dds to jpg and edited them in photoshop. Don't know how to convert them back to dds or where to go from here so any help at all would be greatly appreciated!