Find a post that you really like and want to comment on and scroll down to here:
Click the Reply button and enter your really cool comment.
Press the POST button. WOW! You just made a comment. Lets hope the poster is happy :)
P.S. Also in the quick minnow series:
QUICK MINNOW GUIDE: How To 'Power Up' Your Steem Made Real Simple.
QUICK MINNOW GUIDE #2: How To 'Resteem' Made Real Simple.