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It's the button with the Playstation logo in the center of your Dualshock controller.  This opens the dynamic menu on the PS4. It has an icon that resembles the Playstation logo in a curvy triangle.  Use the controller to navigate to the PS Now app on the dynamic menu and press X to open the app. If you don't see the PS Now app on your main dynamic menu, navigate all the way to the right and select Library.  Then select Applications in the sidebar to the left, and select PS Now in the Applications menu. It's below the Playstation Now icon to the right.  This launches the app. It's the third tab at the top of the screen.  Use the left stick or directional buttons on the controller to navigate to it and press X. Alternatively, if you know of a specific game you want to play, select Search instead.  Select Type to search and use the on-screen keyboard to type the name of the game you want to search for. It's the fourth box in the browse menu.  This displays a list of all available PS3 games. Use the left stick or directional buttons on the controller to navigate to a game you want to play. Then press X to open the game. It's the blue button below the game title.  This starts the game.  Allow a few moments for the game to finish loading.  Press the PS button on the controller to access the PS Now menu. Press the right side of the touchpad to press Start. Press the left side of the touchpad to press Select.
Press the PS button on your controller. Open the PS Now app. Select Start. Select Browse. Select the PS3 games box. Select a game. Select Stream Now.