Follow the below instructions:-
Go to http://in.messenger.yahoo.com/pingbox
Now you will see the following words:-
Chat privately with visitors on your social network profiles, blog, or website through Yahoo! Messenger.
Yahoo! Messenger Pingbox
Click on "Create a Pingbox
To background theme for your chat box, click on any one (Professional,seasonal or colour.
You can choose the colours. For that Click on "Text Size & color: You will see the sizes of 10, 11, 12, 13 and 14
For color click at the box. Then choose the colors whaterver you like.
For displaying on chat box, write something (even your user name or real name).
For onlin greeting, you can give anything. e.g. Hi everyone... You can now chat with me, hello everybody, I am online, etc.
For Offline greeting, you can give anything. e.g. I'm offline. Send a mail to my email address.
If you want nickname for your visitors, click on the box.
Give Pingbox name. Pingbox name wont be visible to anyone except you. You will see it when you sign in using your yahoo id.
After that click save.
Now sign in or sign up option will appear. If you have already yahoo id, just sign in. If still don't have sign up.
Now you will see the html codes of yahoo messenger box.
Now copy and paste the html codes where you want to paste your yahoo messeger chat box.
How people will know you are online?
When you sign in to your yahoo id, you will be automatically online.
look at this image