At the bottom of a post you will see either see 'No Comments' or '1 Comment'
All you have to do is click on that text and the comment box will open.
Type your text...Choose your ID...Type in the two words and or numbers that appear to prove you're not a robot...and that's it...Once your comment has been approved by the owner of the blog it will appear.
To read comments posted by others..Just click on eg. 1 Comment
I hope this helped.