Organizing the Final Scene comes first. Make sure your objects are separated and/or combined with certain others (such as the Hair & Beard on the characters) into objects/object groups. Also, make sure to apply any Modifiers that are being used in your scene.
Make sure each object/group has only the amount of Texture Slots as it needs, using the "+" or "-" button on the Materials Tab to add or remove slots.
Now that all the textures are organized, create a new Material and change the Surface from "Principled BSDF" to "Toon BSDF". Change the color to Black (or whatever you'd prefer). Be sure to click the Checkbox in the Material Settings that says "Backface Culling". (This will be the Outline/Sketch/Toon look for the objects)
After creating this new "Toon" Material go to each object/group and add a new Texture Slot for it. Select the drop-down and choose the new Toon Material. Make sure you remember how many slots each object/group has and which place (1,2,3...etc.) you put the new material in.
After all the organization and material assigning is complete, select all the objects/groups in the scene and reset the Scale, pressing "Ctrl A".
Add the "Solidify" Modifier to a single object. Then select all the objects/groups in the scene again and made sure the object with the Solidify Modifier is "Active" (basically, once everything is selected, click on the object with the modifier while holding shift and it should highlight a different color--indicating "Active").
Press "Ctrl C" and selected "Copy Modifiers". This copies the Solidify Modifier from the Active Object to all the others in the scene.
Go to each object/group and go to the Solidify Modifier. Under "Normals", select the Checkbox that says "Flip".
On the "Material Offset" slider, select the slot in which you placed the Toon Material (1,2,3...etc.).
You should see a Black (or whatever color you chose) outline appear on your objects. Adjust the "Thickness" and "Clamp" to achieve your desired look for each object/group.
I've found the best way to Upload this to SketchFab is to Export as a “glTF 2.0”.
Upload to SketchFab. After uploading, go into “Edit 3D Settings” for your model, and under the “Scene”--”General Tab”, select “Shadeless” in the drop-down menu. (This will set your scene to Base Colors only, disabling certain Material Features, such as Bump/Normal Maps & Baked Diffuse Textures, etc). However, you can add Normal Maps/Baked Diffuse Textures as “Cavity” Maps to simulate some of the texture detail you may lose when using the “Shadeless” setting.
- This method basically doubles the amount of Verts/Faces/Tris in your scene--so it may slow things down for large scenes.
- If you notice any weird shading/solidify effects for an object/group, simply go into Edit Mode for that object/group, select all geometry, and press "Shift N" to Recalculate Normals (this usually helps).