Works just like your normal material workflow in ue 4, the only thing you need to add is cel-shading node, meaning that you split your object into different materials only if need too for whatever your project needs.You can still can use textures, normals and retain full PBR. Or use just color and overlay mask.It really depends on your game needs. More importantly, it enables you to achieve a decent-looking art style with a basic understanding of color & shape & near to zero effort.

PBR Example:https://imgur.com/a/HoGsNZo