The branches of the tree were light against a darker background so I needed to establish where they were and fit the background around them. The twiggy bits were darker so they could be put down on top of the sky where necessary.
When I sketched down by the canal in charcoal I could work backwards and forwards again - the leaves of the tree were light against a dark brick wall - the leaves and branches were 'drawn' with an eraser, lifting the charcoal. Then I could go back in and darken the wall in the spaces.
I really like media that allow me to work this way, pushing and pulling the lights and darks and colours until it's a close as I can get it to how I want.
and with watercolour I let the distant trees and sky blur.
I hope all this makes sense and helps :)