Login to your Google Merchant Center account and click on the "Data feeds" option on the left and click on the "New Data Feed" button.
Choose your target country and "googlebase" as the format type.
Enter your data feed filename. Your filename can be found from the Channels>Google Merchant Center area in your store admin (see image below). Click the icon next to the Feed Name to copy the Feed Name to your clipboard.
Paste the Feed Name in the Data feed filename field.
Click the Save Changes button and continue to the next step.
Create an upload schedule
Your store will generate an updated Google Base feed every 24 hours or whenever manually requested. You must set up a schedule with Google to retreive this feed on a recurring basis for updates. Click on the "Create" link under the upload schedule.
Set your upload schedule, timezone and enter the URL of the file. The URL of your feedfile can be obtained from the Channels>Google Merchant Center area of your store admin.
Click on the "Schedule" button to create the upload schedule.
NOTE: Your file URL must contain the exact domain you used to verify your site with Google webmaster central. If you try to enter a URL file with a different domain, you will receive an error on this step.