Grab apktool

brew install apktool

Download an APK or copy one off your (rooted) phone

Then decompile the apk

apktool d my-cool.apk

Open up the smali source folder and edit the sources

To recompile the apk after making changes

apktool b my-cool.apk

apktool makes it insanely easy to decompile an apk

To edit in java

brew install dex2jar
d2j-dex2jar my-cool.apk

Download JD-GUI

Open jd-gui.app and then pick my-cool.apk.jar

If you have any proprietary information in your app you MUST proguard your app