I am actually writing this out for myself... I tend to forget exactly how I made the last piece - and the subsequent ones tend to come out less awesome. So, if I put this here, I'll remember it. Maybe it will interest others, too.
With a soft textured brush, block swashes of color for the base. This can be very loose and fluid and rough. It should not be detailed.
On another layer, paint on darker colors using same brush in areas you want shadow. Then erase pieces of that layer with an opacity sensitive bristled brush. This will create a rough, oil wash look.
Using that bristled brush, now paint over the lines / edges that look 'digital', and this will add texture to make it resemble a painting.
As an under layer, block in colors and textures that you want to prevail in the painting.
Go crazy. Get creative. But be tasteful.
Now tighten up the fuzzy edges using a super clean (sans soft edge) textured brush. This will make certain subjects and details pop.
When bringing out the details of the main subject, remember to keep it loose, expressive, and intentional. You don't want to rework the expressive brush strokes you just made - just enhance.
Then I soften the white accents with a highly opacity-sensitive paint brush so that the whites aren't so stark and overwhelming.
Since I felt like the ground was so busy, I used that same brush to make a dark wash and light wash over the background... this created a more realistic paint like backdrop.