First of all go to the facebook developers website. If your facebook account is not already registered, you have to register as developer.
After that you are able to create a new app.
Enter the necessary information in the pop-up which facebook is asking for and create your app. After creating a facebook successfully, you will be redirected to the console tab. Here you get your App-ID.
Finally you need to enter the domains on which you are going to use the App-ID. Switch to the settings tab and enter the domain without www or http, e.g. yourdomain.com and also a "Contact Email". After that hit the "Add Platform" button and select "Website". Enter a "Site URL", need to be the same as in App Domains. Finally hit "Save Changes".
Switch to the "Status & Review" tab and make the app public. You also need to start a submission with "user_photos" permission, otherwise your customers can not access their Facebook photos from your Product Designer. Please enter all necessary information that Facebook is asking for. The review of the submission can take a few days.
Optional: You can set icons for the App, that will be display in the dialog box when the user wants to allow the app to access his facebook account. Therefore switch to the app details tab.