From a Home screen, navigate: Apps > Messages. If unavailable, tap the Apps dropdown menu (located in the upper-left) then tap Alphabetical. If still unavailable, refer to Manage App Icons.
Tap the Compose icon + (located in the upper-right).
From the To: field, enter the 10-digit wireless phone number or email address. As the number is entered, a matching contact may appear and can be selected at any time. Repeat this step as necessary to add additional recipients.
From the Add text field, enter the message then tap the Send icon (located in the lower-right).