This is one of the best Photoshop tutorials I have ever seen, especially due to the combination of how good the results are and how easy it is to perform.
Following the steps that are explained in the original post, you will get a grass as natural and real as the one you see in the image above, that anyone could say that it is a photograph of a sports field, right?
The tutorial is very well explained, as always, with text, images and screenshots.
Even to give it more realism, in the tutorial they teach you how to age the lawn and give it a deteriorated appearance.
Source | Tutorial to make natural looking grass with Photoshop