Instructions
Open "Minecraft" and log in to your account.
Open the "Singleplayer" game mode and click the "Create New World" option. Enter a name for your world in the "World Name" bar. Switch between Survival, Creative and Hardcore modes by clicking on the "Game Mode" button.
Click the "More World Options..." button and click on "World Type" until the "Superflat" option appears. Click the "Customize" option and select the "Presets" option to access a list of pre-built worlds. Click the "Use Preset" button and then click "Done."
Leave the "Seed for the World Generator" section blank. Click the "Generate Structures" button until it reads "Off" and then click the "Done" button.
Click "Create New World" to launch the game.
Open the Game menu by pressing the "Esc" button on your keyboard. Click the "Open to Lan" button. Click the "Game Modes" button to switch between Creative, Survival and Hardcore modes. Click the "Start Lan World" button to start hosting a local game. Take note of the code that appears on the screen.
Send the code to your friends so they can join your Superflat world.