You can create a Google Shopping feed in many ways: manually in Google spreadsheets, using data export plugins, a content API, or, the easiest and most powerful method: by using a feed management software like DataFeedWatch.
DataFeedWatch allows you to create a Google Shopping feed in a matter of minutes (with a pre-loaded template) and customize it as much as you need. Once the feed is ready, you only need to copy and paste the feed URL into your Google Merchant Center account.