To flat shade a 3D model in illustration, use 3D software or 2D tools to apply solid, uniform colors to each face/material, removing gradient lighting and smooth shading.
This creates a stylized look where each surface shows one consistent hue—no edge blending—common in cartoons, vector art, or minimalist designs.
For beginners: In Blender, select the model, go to "Object Data Properties," uncheck "Smooth Shading" to reveal flat faces, then assign solid materials. In 2D, import a wireframe render into Illustrator and use the "Live Paint Bucket" to fill each face with solid colors.