Step 1:
Navigate to the Content Section and you will be inside the inbox section.
The inbox is where every product created exists and a the publishing feed is where you can add the products you want to mass publish.
To add products to the publishing feed hover over the 3 dots beside the product and hover over add to feed then click publishing.
Step 2:
Navigate to the "Publishing" Feed on the left hand side of the page.
Step 3:
Click the green publishing button at the top right hand side of the page. A form will pop up.
Step 4:
Select the sites you want to publish the content to, then click the "Publish Feed" button to publish the content to your sites.