Here are some tips on creating awesome cinemagraphs:
1) Start a new animation, make sure you're holding the camera as still as possible.
2) Try aiming your device so that the motion you'd like to see in the animation is near the center of the screen.
3) Make sure your camera is on "auto burst" mode. (The button to the right of the red shutter button.)
4) While trying not to move your device, either hold your finger down on the screen or single tap the red record button.
5) When the movement you're capturing has finished, either let go of the camera screen or tap the red record button again.
6) Tap the green "play" button to begin previewing.
7) Trim any excess animation by tapping on an already-focused frame and choosing "trim after" or "trim before".
8) Tap the little hand/finger icon toward the bottom of the screen to enter cinemagraph mode.
9) Draw on the part of the screen that contains the motion you want to see, making sure to include any shadows. You can pinch and pan with 2 fingers for detailed masking. Tap the eye icon to preview the masked animation (this hides the temporary red mask).