After tapping Choose Shots, first tap the edit button at the bottom of the screen below the shots on your Camera Roll.
Then find the shot for which you want to add a lower third graphic, and tap that shot.
In the photo editing window, locate the keyboard button in the top-right corner and tap on it.
Type your name into the first text field and your title into the second text field.
Then tap Save to save the change you've made to your video.