One way is to use simple shapes. Instead of drawing detailed and complex figures, use basic geometric shapes to represent characters and objects. This gives a more flat appearance. Also, limit the use of shading. Instead of detailed light and shadow effects, use flat colors or just a simple indication of light source.
Well, when it comes to achieving a 'flattening graphic novel' art style, first of all, simplify the details. Don't get too caught up in tiny details like individual hairs or fabric wrinkles. This simplifies the overall look and makes it flatter. Secondly, use a lot of solid colors. Instead of trying to blend colors to create a more realistic look, keep the colors distinct and flat. In terms of the layout of the panels, make them more regular and less complex. Avoid using curved or angled panels that might give a sense of movement or depth. Keep the panels simple and square - like for a more flattened effect. Also, when drawing characters and objects, don't use a lot of foreshortening techniques which are used to create a sense of depth. Just draw them in a more straightforward, two - dimensional way.