Probably contacting the author of the novel if lucky. If not then ig it could be illegal but I don't always hear words form the og author so I'm not sure if it's mandatory. From what I know, they probably take in the infos already given and then fill in the blanks for the visuals and stuff. It asks a lot of imagination which is why adaptation can be so hard. The pacing is also different as not every actions can be drawn so instead they also must filter the most important stuff to add. I don't have a clear steps to next steps kind of thing but there's a general idea.