I'd say firstly decide on the species you're going for: deciduous, fur, pine, tropical, etc. I'm not sure of your skill level, however I'd guess some sketches first. Then you'll need a bark tile texture for the trunk, and some alpha leaves and branches. Look up how folk make regular 3D trees then apply that knowledge to a painterly style. Also research how other hand painting artists produce their vegetation. There's a tonne of nice ones on pinterest to look at.
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