Log in to your Gmail account, and click "Settings," which is in the top right corner of the page.

Select "Labs," and scroll down to "Text Messages (SMS) in Chat." On the right, change the setting to "Enable" from "Disable," and click "Save changes."

Go to Google Chat, and type in a cell phone number where you see "Search, add, or invite." Click the strip that appears to the right and says "Send SMS."

Type in the name of your contact, and click "Save."

A chat box will appear. Any messages you type will be delivered to the cell phone.