1) Download Stencyl Game Creator
2) Run the setup and install it.
3) After setup, launch Stencyl.
4) You can select a pre-built game to start editing it.
5) Or you can Click Click here to create a new game button to create a new project.
6) After that comes your role, you have to select the game genre and some other tricks to implement in your Game (such as: game genre, game characters, game buttons, game playing...). Also it's your turn to make all things that would make your game more original for distribution.
Done! You can save the game as iOS game app. Remember that Stencyl provides also Flash Games and Android games.