This Link is to a wep page that gives specific information on how to create levels in Portal 2, how you can generate great ideas for creating your game and how you can playtest your game to see if it is ready to be released into the public to play and test.

To learn more about the creation of particular types of games a number of tutorials are available on this page. All tutorials have been rewritten for version 8 of Game Maker. They all contain a text document in pdf format, a number of example games, and often additional sprites and sounds.

Link to Gamemaker Tutorials, it has many different helpful guides to how to do different things in Gamemaker.