Click here to see a short video Dahlia Garden
I usually start with a very small thumbnail sketch - it is REALLY small - maybe 2" square - and more of a scribble. Small sketches like this help an artist figure out composition, value & color.
Then a more complete drawing is done and transferred to watercolor paper.
In the example I'm showing here, I painted a 'resist' liquid on those areas that needed to remain white (or protected) while I did background color washes.
Once the background was complete, I removed the resist and could paint those items (barn & flowers) that were directly painted against the background.
Gradually the brightest flowers were painted and then on to filling in the foliage, adjusting values along the way.

Comments
congrats on your commission, I hope it went well.