The main dev of the game makes devlogs about the game. This is the first one with a time-stamp to the cell/toon shading bit:
He mentions a tutorial that he used, which can be found here:
Although it’s for Unity, you can use it as a guide. Also, this could be helpful:
Roblox recently added the Hightlights feature, which has it’s flaws, but could be what you’re after: Highlight | Documentation - Roblox Creator Hub