The main workspace — the first item in the sidebar — is where everything lives. Click Add site and enter a name and the site's domain. Nothing else is needed: there is no per-site configuration to write and no selectors to maintain.
Each site appears in the tree on the left. Selecting one shows its products, prices and matches in the table, so you can work through one shop at a time or compare across all of them.
Each account can track up to 3 competitor sites. To track a different shop once you have three, remove one first. Discovery, price refreshes and matching on your tracked sites are unlimited.