If you use anything that is not yours, ask first. A lot of time goes into making a theme (even a not-so-good one) and I've seen too many people just say "credit goes to blah blah" but never asking. Please be considerate of the work of others.
As for actual theming it all depends on what you are theming. Some apps (launchers and widgets mainly) have built in theme functionality and should have a guide specific for them. Theming a ROM is a lot more time intensive and involves decompiling frameworks, setting up development tools, dealing with many possible problems, hoping compiling works, being frustrated about forgetting something, testing, etc. etc.
• Minimal Pandora •
T-Mobile Galaxy S III
• Paranoid Android 2.23 •