iPhone and iPad owners can now view available pre-order titles in the Game section of the Apple App Store. Currently there are only a handful of titles available, including the music rhythm title, Cytus II, Thumper: Pocket Edition, Full of Sparks, JYDGE, Slime Pizza, and Lord of Dice.
To purchase any of these titles in advance you must navigate on your device to App Store > Games > Pre-Orders, then click the title you want to pre-order. Tap the blue bubble featuring the app's price like you would do when purchasing any other game on the App Store. You will then receive a Notification when the game is ready to download, or the pre-ordered game will automatically download within 24 hours of its release on devices set to automatic download.
You can enable automatic downloads in Settings > iTunes & App Stores. Just tap the "Apps" slider under Automatic Downloads so it is ON/GREEN. To disable automatic downloads tap the slider so it is OFF/WHITE.
You will be charged the lowest launch price when the game is released. For example, if the game is listed at $2.99 at the time of pre-order, but has a limited time launch price of $0.99, you will be charged the lower price.
You cannot pre-orders games from your computer. You must pre-order them through the App Store app.
At this time there doesn't seem to be a Pre-Orders section in the Apps category of the App Store. This is probably due to a lack of non-game apps being available for advanced purchase. Developers will most likely list more games for pre-order since mobile gamers will be the primary users of this new feature.
Apple announced in mid-December that it would begin allowing developers to list games for pre-order on the App Store.