Source: http://www.wired.co.uk/magazine/archive/...

"Developing apps from scratch is time-consuming," says Vojtech Svarc, founder of StartAppYard, which sells modifiable app templates -- such as Flappy Crocodile, based on Flappy Bird, the intensely addictive mobile game. "Just look for a template you can use and modify." Then follow these steps.

1. Get a developer accountYou'll need this to publish the app. Svarc used Apple Developer, which costs $99 (£59) a year. You'll also need a program to modify source code, such as Xcode, which you can download from Apple. "You don't have to be advanced, but you should know your way around code," says Svarc.
2. Buy source codeThese cost between $99 and $349 and can be purchased from various websites, such as startappyard.com, apptopia.com orchupamobile.com. Svarc recommends novices begin with his Flappy Crocodile code, as it is popular, addictive, and has a step-by-step developer tutorial.
3. Change the game"Changing sounds and images is a bare minimum," says Svarc. Overwrite details in Xcode by dragging and dropping the new files over the original source code files. You can monitor your game's progress with "product clean" and launching the game to play it in a simulator.
4. Upload and publish the appIf you plan to sell your app, change the advert settings to ensure you are promoting your own app. Change the name and other identifying info in Xcode. Before you submit your code to the App store, ensure that the file is below 20MB -- any more will take up a substantial amount of space on a mobile device.