The first thing you want to do when you have such a question, is to google the topic (not very useful in this case - most of the links seem to be to various sorts of "Sim-Bonsai"
)
Many of the best at virtual bonsai, simply use an image editing program, such as Photoshop (there are many less expensive ones) and master the techniques needed to use the program expertly.
Such a discussion would be far beyond the scope of this forum. The books that instruct are
heavy!! The time needed to learn the techniques is long.
And then, you have to know how to style bonsai!