It's pretty simple. Just click on a tool to select it, and click anywhere on the screen to place a block. The lava kills you, and the purple block bounces you higher. Press Q to delete blocks. Press C to clear your level. Press Play Level to test your custom platformer, and have fun!
I give credit to IronBizMark, a great scratcher and one of my friends in real life, for the simple physics script I used. Everything else was designed and programmed by me. In the future I will make it possible to share and save levels. For now you have to show the game to your friends the old-fashioned way. Update 1.2: +Grid Button +Clear Level Key