The cross is the first, and probably the most fundamental, step in CFOP method. This step is where you gather the four edges of one side onto that side and put them into their places. I say fundamental because most people can do this intuitively - given enough time - and start to learn how pieces move around the cube. The cross probably doesn't need a tutorial, but i do have some moves listed here for things that might annoy some beginners, like flipping a piece in place, or swapping the cross pieces around after you have made a cross but notice the side colors don't match.