Here are the following instructions on how to enable iCloud Automatic Downloads for Apps in iOS. However you will need an iTunes account to enable this new feature.
In order to enable Automatic Downloads for Apps, iTunes, or iBooks turn the respective switch to the ON position.
If you want 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