From a Home screen, navigate: Apps icon > Messaging.
Tap the Menu icon (located in the upper-right).
From the Multimedia (MMS) messages section, tap (check) Message signature to enable or disable.
If not available (greyed out), tap Messaging Disabled (located at the top) then tap YES to confirm.
A signature is automatically attached to each sent text message when this setting is enabled.
If enabled, do the following to edit the signature: