Theory
A brush without settings is useless. The brush is supposed to do all the work for you, so you have to set it right. I bet you've all seen examples of art where someone used the brush with the same opacity and size all over the picture, looks fake and out of place.
The brush has to have different sizes so: tablet- shape dynamic on, mouse- just change the size often.
Change the colour of the grass with each row you put or else no one will be able to see any volume, just a one-color stain.
Don't use the full opacity, grass is transparent to some extent.
Scattering is crucial, seriously, you do want to scatter your brush, both axes, I