Getting the feed there is simply a matter of bulk editing and making all of your items available on the google shopping channel. You do that from your products page. It is a 5 minute task. Be sure when you bulk edit you don't move to the next page before the first finishes adding the edits. Once you get to google it will be a couple of days before everything is verified. Don't be hasty to edit on the Google page. Let it sort itself out then you may need to address any issues.