Facebook Chat is so easy as following this step by step tutorial.
How to Disable and Remove 'Seen' in Facebook Chat ( Google Chrome )
1) Go to Facebook Unseen Extension for Chrome
2) Click Add to Chrome
3) Click ADD to confirm download.
4) The extension will be added automatically.
5) Now just navigate to Facebook.com
6) Start chatting and you'll notice that 'Seen' was disabled.
Done! If you want to disable this plugin, click on it (Right top on Google Chrome) and Click OFF