Many ways. Easiest, just find it in app store and download it. Apple knows you already paid for it you'll see a pop up notification will say You have already purchased this app would you like to download for free.
Next icloud is rolling out, you can browse thru all your purchased apps that aren't on what ever device you're on. Also set it to sync automatically any new apps. Apple.com has all the info on icloud. Read up, good stuff coming
Download it onto your iPhone using The same account you used for your iPad. It will still say purchase but once ou put in our password it will have a pop up that will tell you that it is already bought so the download is free.