A few days ago, we covered a tool aimed at making it easier to use Baksmali/Smali, a disassembler/assembler for the Dex files used by the Dalvik Virtual Machine in Android. Another incredibly useful tool to have in your modification arsenal is APKTool, which was originally developed by XDA Recognized Developer Brut.all and is continued to this day by XDA Senior Member iBotPeaches.
While APKTool is incredibly powerful, it can also be intimidating to new themers and modders. Thanks to a guide created by XDA Recognized Contributor and Themer PulseDroid, though, this should no longer present much of an issue.
The introductory-level guide covers what files you need, your development environment, and how to actually use the tool to decompile and recompile. This is accomplished in four easy steps: installing framework, decompiling (and applying your modifications), recompiling, and inserting the new contents into the old APK to retain the old signature.
The guide steps are accompanied by copious screenshots and plenty of explanation. All in all, this incredibly easy-to-follow guide will get you on your way, regardless of how little experience you may have at the start.