I am the last person who should reply since I have absolutely no
artistic talent. But since nobody else has chimed in with suggestions
I'll do my best to help.
I use Copic markers,
which are alcohol based and the only markers I've had luck using when I
want to color an image. I did a quick YouTube search and learned that
your Zig Real Brush markers are waterbased. It looks like they work best
using watercolor paper with a paint brush or water pen. Here are a
couple of examples:
https://www.cathyzielske.com/2018/01/watercoloring-zig-clean-color-real-brush-markers-free-printable-swatch-chart/
Shading
and highlights are difficult for me as well. I have to imagine the sun
or other light shining from a corner of the page. Hopefully you'll find a
better explanation in one of the links below:
https://www.artinstructionblog.com/drawing-lesson-a-theory-of-light-and-shade
Links below as well. Sorry about the formatting. I can't figure out how to fix it.
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