I'm only a dabbler in UI skinning (I don't have the artistic talent for it), but here is my understanding of the process:
1. Get the official U14 skinning pack
, which is the latest one that has been released by Turbine.
2. Take the SkinDefinition.xml file and remove all of the lines for images that you do not
wish to change. This file should contain only the things you want to change
about the UI, not the things you want to keep the same.
3. Edit the .tga files as desired, using Photoshop or GIMP. Remove any .tga files you don't want to modify. Your skin only needs to contain the .tga files that are different from the defaults.
4. To change the size and positioning of things, you need to find the appropriate section in the SkinDictionary.txt file, and copy+paste it into your SkinDefinition.xml file, then modify it as desired.
5. Ask Adra.