How to Make a Game App for Free
Make a game app for free by modifying the source code of an open source game. The Web is filled with source code and many other resources to help you create your own game programs. Another type of resource for game development is an integrated development environment, which helps you write program code that conforms to the syntax of the language you're writing in. As you type your game's programming statements, write comments explaining what each statement does. This makes debugging much easier.
Download the source code files of an open source game from Source Forge, such as Rigs of Rods, Frets on Fire, or Assault Cube. Open source game developers typically make source code files available in an archive file, such as a ZIP, RAR, or TGZ file. The source code within these archive files comes in plain text files that have extensions such as ".c," ".java," or ".bas."
Download a software development kit, or SDK, for the language mentioned in the instructions that came with the source code's archive file. If the game's programming language is C++, you can get an SDK from Microsoft, GNU, or Open Whatcom. For a Java game, get the Java Development Kit from Oracle's site.
Read the program's instructions for compiling the game from the source code, then read your SDK's instruction for compiling. Compile the game from its source using the SDK's compiler, then run the game.
Take notes on things you'd like to change about the game. For example, you might write that you'd like to make the game's characters look like cowboys instead of people in spacesuits.
Open the game's source files in a text editor such as WordPad, then read each statement in each of the source files. Write down notes explaining what you think the statement's purpose is. If you don't know the statement's purpose, write a question mark or "Don't understand yet."
Read the documentation of your SDK to learn how to use its tools for executing a program one statement at a time, which some SDK's refer to as "step-through" execution.
Run the game one statement at a time, jotting notes on the game's behavior and the current statement being executed. Update your original notes with the new ones, especially the notes that show question marks or "Don't know yet."
Retype any one statement whose purpose you understand, then recompile the game and run it to see the effect of your change. Make several more changes, using the list of changes you wrote as a guide for what to change. Once you can no longer recognize the original game you downloaded, you'll have made your own game application.