There is no complete guide to skinning, your best off looking through this forum for skinnig questions, aslo sortitoutsi's forums have a couple of guides in the guides forums (recolouring and slicing).
http://smithjurdskins.co.uk/ also have some simple guides.
You'll need a image editing software (Photoshop, Paint Shop Pro, Fireworks, The Gimp any of them should be fine). You'll also need something to edit the xml files with, you can use notepad, but I'd advise you to use an webpage editor, one with line numbers, and code in colours - makes editing a whole lot easier.
I'd suggest first downloading the Gold Demo if you don't have the game.
Make a copy of the metallic2006 skin folder in data\skins
Then edit the files inside the copy of the folder you made.
The best way to find out what some graphics do is recolour a sub-folder at a time into a bright colour, which you can not miss, and then record what each subfolder controls.
To edit the placement of stuff, you'll need to edit the xml files in SKIN_NAME/panels.
header - controls the header.
footer - controls the footer.
sections - is the sidebar on the left/right
match title bar - header in match.
When you open the xml files, you'll see loads of code.
I'd suggest adjusting just one line at a time until you know what it does.
I tend to adjust the alignments by about 100 to see what they do and where they are, you can also use -ve numbers. Once you know where the item is and how to move it use small changes to position best.
I'd advise you to just spend some time playing around with the files, so you learn what most of the stuff means.
And then come back with specific questions, they will hopefully get better more detailed answers on how to do it. (If anyone knows).