First to you need to install the app from this link :
After you done the installation you need to connect the app to your Twitter Account by clicking on Connect to Twitter button that will redirect you to Twitter.
After you connect your Twitter Account you will redirected to the Twitter Setup Interface to setup the app. On this Interface you must set an custom message that Social Feeder will use during tweeting the news products of your Shop. Click now on Save button to save your custom message.