Click on its timestamp to go to its page, then click the 'flag' link at the top. There's a small karma threshold before flag links appear.
" This could be more clear. I was able to figure out what they meant, but "25 minutes ago" is not a timestamp. If you hover over the link it will display the timestamp, but I never even noticed that until today.
I think it would be clearer if the text said "click on the reply link". Yeah, I know the link doesn't say "reply", but "reply link" is what it is, and "timestamp" is something it isn't.