Create a line sketch
I always start with a loose line sketch on its own layer, to help block in the locations of key items in the composition. If working on an animated film this might be defined from an unlit production frame, but I still usually draw a simple line sketch over the render so I can have it for easier painting.
Resist local colour thinking
The most common mistake I see with new artists is to only think about local colour (trees are brown, leaves are green, grass is green, sky is blue, rock is grey, water is blue) and then only paint lighter and darker variations of those local colours to mimic light. Don’t get trapped! Light is a powerful tool, so try to throw out your assumptions about local colour and only think about light.