How to reverse engineer an apk

Two steps:

1. Rename apk to zip and use 7zip or any other utility to extract the contents. Inside it there is a dex file. Use this dex2jar tool to convert the dex file into jar and then use JD java decompiler to decompile the jar file.You will get almost all the codes except for the xml files, like manifest and layouts. For those follow the second step.
2. Use apktool.