You can add a comment to a bug directly from the details of that bug.
Below any attachments, there is an Add Comment header.
Add a Subject line and type your comments in the Body field. Check Use HTML in Body to format bug details using HTML code.
If you wish to provide a link to a specific Bug or a Discussion message within your comment, use Link Notation.
You may also choose to add an attachment at this time.
Alternatively, you can enter comments about a bug when you modify a bug.