Creating an FTP account
From Websites & Domains, select the domain then FTP accounts. The main account (created automatically with the hosting) has access to all the domain's files; you can create additional accounts with more restricted access.
For a new account, enter a username, a strong password, and the root folder that account will reach — it won't be able to go above that folder.
Restricting access to a subfolder
That's the most common use of secondary FTP accounts: giving access only to one specific subfolder (a site's theme, say, or a particular upload folder) without exposing all the domain's files, including sensitive configuration files.
Connecting over FTP
The connection settings (host, port, credentials) are shown directly in the detail of the FTP account created. Use a standard FTP client (FileZilla, Cyberduck) or your editor's FTP integration.
Deleting or disabling an FTP account
An unused FTP account should be deleted rather than left active indefinitely — each active account is one more potential way in to the domain's files. Deletion is immediate and doesn't affect files already on the server.