Howard Pinsky shows us how in this short video.

see to it the Timeline pops up on the bottom of the screen.

If it doesnt click Window > Timeline and you should be good to go.

Head over to the Layers Palette and drag the duplicate to the top of the layer stack.

ensure it is right above theVideo Group.

Right press the duplicate layer in the Layers Palette and select Rasterize Layer then a Layer Mask.

Press B on your keyboard or choose the Paintbrush Tool.

(Pinsky used a 200px brush with 0% hardness at a 100% opacity.)

Press Spacebar on your keyboard to preview your cinemagraph.

Hit the Export button on the bottom left of the Timeline (It looks like a bent arrow).

Name the file, grab the h.264 file format, and press Render.

Hit the link below to read more.

Learn to Make a Cinemagraph Using Photoshop in Under Two Minutes| DIY Photography