Default Snapchat.apk Code Injection

I have a little question to snapchat. Currently I am working on code injection into an apk. As an exercise, I have chosen Snapchat. My goal is that I insert a small button and when you click on the button, the video / image is saved. How can I insert a button into an apk and how is it possible that you can save the current image / video? I know how to dekompile an apk and edit Smali files but I dont know how I should do that in Java. I Font know how I should begin.

Thank you