(1) Open the fact on which you want to comment.
(2) Below the fact there is an input mask.
(3) Insert a title and a text. If you want to be notified when somebody replies to your comment, keep the checkbox checked.
(4) Click on "publish >>".
If you cannot see the input mask "Add your comment", the feature either has not been activated by the p-admin or you are not permitted to comment on facts.