Choose the shot you wish to caption by clicking edit and then your selected video clip.
Click the keyboard icon in the top right corner.
Click the icon to the right of "Cancel" to toggle between the bottom-third caption and full-screen caption.
Type your caption and then click "Position" to resizes and reposition your caption.
Zoom and drag with your fingers to resize and reposition your caption. Once you're satisfied with the caption, click "Save."