Step 1:
Either press the new message icon or search for a contact using the search bar.
Step 2:
If you press new message icon you will first have to enter at least but not limited to one contact, if you used the search bar you can just click on the name you wish to message. To remove a name press the 'X' after their name.
Step 3:
At the bottom of the tab is the text field with 'Aa' or 'Type a message...' if in full screen mode. Once you have type your message press the 'Enter/Return' key to send the message.