Data Feed Optimization starts with exporting all product data from your store. There are four ways to do that as different shopping carts (ecommerce platforms) have different methods:
Export to desktop
Some shopping carts enable you to export all product data as a CSV file (or other file type) that you can download to your own desktop. This is a quick and clean method and you can upload this file to a data feed tool or modify it and upload it to a channel. The downside of this method is that this "manual labor" needs to happen every day, to make sure that the data feed is up to date.
Export by a shopping cart app
Some shopping carts offer applications that enable the merchant to export his product data in the channel's format. For example: Many shopping carts have an app or extension that provides a ready-to-go Google Shopping feed that contains all product information according to the feed specs of Google. They may have a similar app for export to other channels.