1. Open Yahoo Messenger window. In the menu, go to Actions -> Send an SMS Message…
2. You can select an existing contact with a mobile number from My Contacts tab
3. Otherwise you can type in the mobile phone number in Other Contact tab. The number should be typed in International Telephone number format. That is, start with a + followed by country code, area code if any, and cell phone number. In India, it would be +91 followed by 10 digit mobile number.
4. You get a new SMS window, just like your regular Yahoo chat window. Type in your message, not more than 143 characters and hit Send. You can send the message from any of your Yahoo profile identity.
In order to continue the conversation your friend need to reply to the message from his cell phone.
