Press the flag twice, and wait for it to load. Arrow keys to change blocks (the pickaxe mines blocks). Click (and hold it down if you want) to place blocks. If you move the mouse to the top right of the screen, you will see a "Random" button. It makes EVERY SINGLE BLOCK random! It will not work on just one or two blocks! It does ALL of them! This is sort of like Minecraft. Comment which blocks you want me to add (no partial blocks [torches, flowers etc.] or things you can't place [steak, bone meal, etc.])!
Thanks to Mojang for coming up with Minecraft! I did the rest.