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. :)
A special commission for a Christmas present, drawn on A3 medium cartridge using 0.03, 0.05 and 0.1 fine liners. This was a bit different for me but I loved it so much! Took about 28 hours in total.
I drew these guys with a Pilot Custom 823 FA and Platinum Carbon Black ink. The FA nib has a good drawing width on watercolor paper - one of my favorites. It's a soft nib but I don't use the line variation, just some nice shock absorption with each downstroke. The Carbon Black ink is out-of-this-world good, though being pigmented I fear keeping it in too many pens.