I stated with a rough pencil sketch then inked it with a brush pen and colored it with watercolor and gouache on a watercolor postcard. Reference I used was this excellent photo - https://www.pexels.com/photo/tree-wild-squirrel-monkey-97827/ photo by Mike from Pexels.com
Here is a painting I made from observation on the top of Mont Bélair in Canada, just before sunset.
I am painting those in a 5x8 very convenient moleskin watercolor book. I previously eyeballed the dimensions of this book at 4x6 when I had no ruler to verify but I was slightly wrong. Now the info is exact. :)