From the Start screen, swipe left to display the apps list.
Alternatively, tap All apps (located in the lower-right; may require scrolling to the bottom).
Tap New message (located at the bottom).
From the "To" field, enter a 10-digit mobile number.
Alternately, enter a contact name. As the name is entered, a matching contact may appear and can be selected at any time.
Alternately, tap the Plus symbol (located at the end of the To: box), tap a contact then tap the appropriate mobile number (if applicable).
Repeat this step as necessary to add additional recipients.
From the "Type a message" field, enter a message.
Tap Send (located in the midle; at the end of the message box).