First, you need to choose a public domain book. Then, break the story into scenes and dialogues. Next, find or create suitable art for each scene. You can use free or cheap digital art tools. After that, use a visual novel creation software to assemble everything. For example, Ren'Py is a popular choice. Finally, test the visual novel to make sure the story flows well and the art matches the mood.
Well, for converting public domain books into visual novels, one important step is to script it out properly. This means taking the text from the book and formatting it into a visual novel script with dialogue, character actions, and scene descriptions. Then, in terms of the visual aspect, you can look for public domain art that might fit the era or style of the book, or if you have the resources, commission artists. Another key thing is to make the user interface intuitive. People should be able to easily navigate through the visual novel. And don't forget about proofreading and beta - testing to catch any errors in the story or functionality.