GuiAPI uses the TWL library from Matthias Mann (probably spelled his name wrong), see twl.l33tlabs.org

• First decompile the needed minecraft classes with jad, then run them through astyle --break-blocks --delete-empty-lines --style=java
• Apply the diffs in diffs/ to your decompiled minecraft and copy those java files to src/
• Build (I use eclipse)
  • You need .minecraft/bin/*.jar, Modloader, twl/bin, and xpp on your build classpath.

To create a distributable archive, package twl/bin/*, xpp*/*, theme/*, and bin/*.

Lots of people who I forget. Open an issue or something if you helped ...