Export → Bake the textures :
As precised into the introduction of this tutorial, the export of Cycles shaders to Godot is not possible for now. But there is still a way to "translate" these same shaders so that Godot integrates them, here is how I proceeded :
First, we must bake the textures and for this I partially used the method described by Aidi Burrows from CG Masters :
Into this video, it is said that a baked image allow the reduction of the rendering time. Aidi indicate that, in his case, it take 1mn + 25 seconds instead of 5 mn + 20 seconds waht is already amazing for a simple mesh like a planet, so we can imagine what it would give for a more complex object !
As reference, I will use the sun created here.
Once baking operations done, we can notice on screen that that colours and Voronoï texture used for the shader are. Obviously, the emission shader is not, but this one is perfectly reproducible into the game engine.
The export itself :
For my first tries, I opted for the export with .obj + mtl files but it did not work. I so used *.dae format.
For this we have to download and install Better Collada into Blender from Github. Indeed, the Collada exporter by default for Blender would not work properly.
!
! 
Note : Don’ t forget to activate the plugin into Blender Users Preferences.
Import into Godot :
Place the obtained *.dae file into the project folder as well as the diffuse texture previously created/baked (yellow.sun.bake_diffuse.png in our current case).
Open the project then load the scene to edit.
Note : For more clarification, I created sub-directories (missing in the first chapter) into the project folder.
You should have something like this on screen :
! 
Rather than letting .dae and .material files, we will re-import the mesh to lighten the number of files, which normally and gradually, should lighten the entire project:
- 1) Select the *.dae file into the resource panel
- 2) Click on the Import tab
- 3) Rename if you want
- 4) Into the Meshes part, go to Storage and select Files
- 5) Click on Reimport
! 
- 6) Drag and drop the obtained .mesh file into the viewport at a desired place. A new MeshInstance node will be automatically created
Note : Here, the .mesh is named Icosphère.01 because it’s its actual mesh name into Blender. We can obviously rename all of this without problems (right click or left double-click).
! 
Now, we can absolutely remove .dae and .material files as well as the corresponding *.import files (if still any) into the project folder. And by the way, let’s rename the files for more clarification.
- 7) Go into the Inspector → Material, then select New SpatialMaterial
- 8 ) Select Edit
! 
- 9) Drag and drop the diffuse texture (here : yellow sun_diffuse.png ) into the Albedo part and the field noted as « null »
! 
- 10) Then into the Inspector, go up to Flags and activate Unshaded
! 
Save this new resource as *.material file and save the scene. We are now done about the import !
Convert to ShaderMaterial :
Nothing more simple :
- 1) Go back to the MeshInstance edition (simply click on the node into the Scene dock)
- 2) Go to Material, then select Convert to ShaderMaterial
! 
We have now a new shader as material, great, but how edit it ?
- 3) Select Edit
- 4) Select Edit
!
! 
Save this new resource as *.shader file.
Concerning the shaders edition itself, it will maybe be the topic of another tutorial (but no promises).
Of course, the whole procedure would have to be repeated for each added object.
Downloadable PDF version
If any mistakes noticed, please let me know