When you go the Contact Record you will see the option to Send Text in the phone number field. ( If the contact does not have a cell phone number listed you will not see the Send Text link.)
Click on Send Text.
This will take you to the Text Messaging Assistant screen where the system will automatically choose the cell phone number on the contact and allow you to add your message. You will have the option to choose a different cell phone number in the To drop down if there are multiple cell phone numbers for the contact.
Once you have typed your message in the Message box click Send. You will then be returned to the main screen of your Contact Record.