To send an SMS, your contact needs to have at least one valid phone number. Then select SMS in the communication tools.
Note that you can't send an empty SMS.
You can type your SMS or use canned messages, which are ready made templates, by clicking
Once you have some text simply press Send
Multiple phone numbers
If your contact has multiple phone number a small triangle will appear next to the main phone number and you will be able to click to select the phone number you want to send the message to.