Android - How to decode and decompile any APK file?

Question

I\'m working on ads: my customers gave me some APK files of their Apps.
My work consists of inserting ad banners into them.
After closing the ad banners, these apps

Answer 1

To decompile APK Use APKTool.
You can learn how APKTool works on http://www.decompileandroid.com/ or by reading the documentation.