Press to launch Settings from your Springboard.
Press to select Store from the Settings Menu.
Press the Sign In button if you are not already Signed In.
To enable Automatic Downloads for Apps, iTunes, or iBooks turn the respective switch to the ON position.
To enable Automatic Downloads while you are on the cellular network turn the Cellular switch to the ON position.
Now when you purchase or download a new app, song, or book the item will automatically be pushed to all your devices.