Check out the associated course "Stylize Renders with Post Process Materials" on the Epic Dev Community.

In this course, you will explore Post Process Materials as a way to create stylized looks at render time using PBR shaded meshes and standard lights. Kris Kauffman shows how the UE4 render buffer can be manipulated to create looks that simulate unlit flat shading, “toon” outlines, and a painterly look. Then remix the techniques to create any look you might imagine.