The brush shape is not that important! Yes, it is not that important.
The foundation and step are the real things!
Foliage and bushes are very important for background painting.
With creating some asset of foliage, you can copy and paste it in the
background as filler.
Using multiply and a few layer effects for the shading and shadow is good
enough.
But How to make good foliage that matches all your future projects?
Here I have a quick video record of how I make foliage.
I'm using paint tool sai for this painting.
I'd like to make a video with voice over next,
just to explain my secret tricks to you along with some asset
included if needed.
Hope you enjoy my quick video on how I make foliage and bushes.
youtube post: tw5oBZcW86c