How can I do this?

http://www.youtube.com/watch?v=5wxe1IUu5QA&feature=relmfu

at 0:36

Look at the lines on the ground. How do I do that without creating a new scene with demo island.?

there is no need for you to create the line it is already built in unity you can start from creating the floor using cube.that is the surface where you will place all your game objects in every game when you start anew scene it will be there

You mean the raster spanning the x and z axis so you can see the 'ground'? I don't know. I think it's only on unity pro. But you don't really need them so don't worry. :)