when you want to pay you will be redirect to a different site and you had to fill the payment details and after its verifed you get a expansion pack in the main site
There's a tutorial indeed, but it's based on the PC version, in which all the paid stuff is unlocked (since there's no free version of it). Which means that making a version from it and releasing it would be piracy, so I suppose you don't want to do that.
You'd have to start from the code that was used to generate a mobile version, but those are obfuscated.