This is a very experimental stage, where we manipulate many attributes and combine values to get a realistic image.   This part of the process is largely trial and error; sometimes the default values will work adequately, but you will likely want to change quite a few values.  I will explain how to manipulate the shape and form of the grass, as well as what each attribute modifies.

You can see explanations of every attribute if you browse your help panel

You can see how I manipulated my settings to create a thin cross-section at the top of the blade and a wide one at the base.   I also changed the density settings, as well as the thickness of the branches and the little lefts that are in the end of the large blades' branches.  I also changed the elevation angles so that I could simulate gravity or wind.   A little twist in the branches and some variation in the size and length of each blade and branch also aids realism.  Keep experimenting with settings until you are satisfied with the look of your grass