How to make mod applivations in androif

Answer 1

*How to make modifications in android application?
-> Reverse Engineering
first
You'll need to have knowledge of android assembly language
After, you can use various tools like
apktook-> to modify Smali code
For source code inspection only:
winrar -> to extract apk
dex2jar-> to decompile classes/2.dex
jdgui-> to read java files in jar archive