Wrong forum. This belongs to Texturing & Surfacing.
Moved.
There can not be such a thing as a tutorial for custom User Interfaces.
It's 2D Art. What
can be done, is explaining the mechanics.
You will find
67Chrome's UI Template quite helpful, aswell as our
Texturing Contest #18: UI.
Creating a freehanded custom User Interface takes a huge amount of time and effort.
The most important thing is a coherent concept.
You can decrease working time by copypasting images together. However, make sure you have permissoin to use the chosen images and cite the sources.
Any UI consists of at least 6 parts:
Tilesets 01-04, TimeIndicatorFrame and InventoryCover.
Look at Warcraft 3's original ones to check out their resolutions and alpha channels, should you want to work on your own UI.
Additionally, there are Menu and Esc Borders (the latter used for Cinematics). These are looping textures (some parts repeat themselves, because it's impossible to have one defined texture for borders of different sizes).