The restoration steps differ depending on which app market you made the purchase from, which in turn basically depends on which type of device you own. Please go to the page appropriate for your situation:
- iTunes (purchases made for the iPhone, iPad, and iPod Touch)
- Amazon (purchases made for the Kindle Fire family of products)
- Google Play (purchases made for all other Android devices)
Please note that purchases from one app market are restorable to other devices that use the same app market, but do NOT transfer to devices that use a different app market. I.e., you can't apply your iTunes purchase to a Google Play device, or vice versa. You can read more here about why this is the case.