After you log in to your FastPanel, open the Site Card of a website you wish to generate SSL for. Now find an option named SSL certificates:
A new window will open with all the SSL certificates that exist. In this case, there are none. Click on a blue button named New certificate.
In this window, LE will already be selected, and all the info needed will be prepared.
Wait a few minutes, and you should see a lock in a green circle and below it will say Valid. This means that the certificate was generated successfully. LE certificates last for 90 days and will be automatically renewed.