13), 13a) We've named the layer, Static Frame.
13b) Move it to the top of the layer stack. 14) Time to mask. Depending on your skills with layer masks and the actual portion of the frame you want to animate, you can mask in different ways. We've kept it simple here. The fluttering flag moves around quite a bit, even in this short clip. Because each frame in the clip is perfectly registered (see earlier note about using a tripod to lock down your camera), we can just use the elliptical selection tool to set the area where the flag will flutter.
14a) Feather the selection to keep any oddities between frames from being obvious.
15) Make the Layer Mask. Go to Layer > Layer Mask > Hide All.
15b) Our layer mask on the Static Frame Layer. When we hit the play button (bottom left on the Animation Pane), the underlying layers will run under this Static Frame and the animation will show in the masked area. Select Forever in the Animation Panes playback controls to see the whole effect. The biggest problem most people have with a cinemagraph is having the animation show smoothly when it loops around. Choose your static frame and your last frame in the sequence carefully to avoid a skip. In this case, we selected a particularly challenging subject and it won't be perfect.
16), 16a) Once you've tested and you're satisfied, it's time to export your cinemagraph as a GIF. Go to Save for Web & Devices and under presets, choose GIF 128 Dithered. And that's it. Your finished cinemagraph is ready for posting.