The fastest way to do this is on under the Followed Companies tab. 

On this tab under Your Followed Companies, you have several options to begin following companies. Owler now gives all users the ability to create custom lists of companies, Save advanced searches, or for Max customers even sync lists from your salesforce account.

On this page you should see the first list Your Followed Companies. This is Owler's out of the box list for all community, Pro, and Max users. If you click See more, you will see the ability to quickly search for companies to follow (We recommend searching by website or URL) the ability to import a list of websites to quickly follow hundreds at a time.

You can also unfollow any company that you don't want to follow anymore on this page by clicking the boxes to the left of the company names that you do not wish to follow anymore, and then clicking "Unfollow companies." 

Another very simple way to follow or unfollow companies, especially if you are just searching or browsing, is from a company's Profile page. Just click "follow" at the top right to begin following, or clicking on the "following" button to unfollow.