Initially, you would have to find the Google Shopper application in the Google Playstore in your Android phone. After finding the application you would have to download it through usual process if downloading an application in an Andriod smart phone. When the app is fully downloaded, it will go through installing process after which it will be ready to be used.
When you have finished installing the app, go to the home screen and find the icon of the app and touch it. The app will open ask for the type of input you want to give including text search, voice search or barcode.
As text search and voice search a usually familiar to an Android user, we will detail the barcode. First you have to click on the camera icon located in the homepage of the app. When you click on it, a line will appear on the screen in which in you will have to zoom the barcode of the product once the barcode is recognised by the app it will give all the details of the product. A list of stores will be shown where the product is currently available.