It's pretty easy, just keep a few things in mind:

1. The scales flip on the X axis. So when creating the custom scale, it should be oriented along the +Y axis.
2. Try to keep the poly count as low as possible, to keep your computer from exploding
3. Don't add thickness to your custom scale object. It needs to be a flat plane to be processed in the geometry nodes (To make thickness and procedural glow settings work)

Now to the creation of the scale:

1. Add a plane to your scene
2. Put the origin of the plane at the end of the plane (The one that points into the -Y direction)
3. Shape the plane as you like: Add loop cuts, bevel it, delete faces, etc. and don't forget: The +Y end is the tip of your scale
4. Delete the UV map: Properties Panel --> "Data" --> "UV Maps" --> Press the "-" symbol on the right
5. Select the custom scale in the Mystique geo nodes modifier as "Custom Scale Object"
6. If the scales are way to big: resize the custom scale object and apply the scaling by pressing "Ctrl+A" --> Scale

Take a look at the modifier settings and see if the checkbox "Fast Preview" is checked. If it is, uncheck it. Don't forget to uncheck it befor rendering!

Yes! Just add the shader node group "Mystique Effect Shader Control" to every material on your object. The rest happens automatically.

1. Click on the object you want the Mystique effect on
2. In the properties window go to the Modifiers tab (wrench symbol)
3. Press "Add modifier" then "Geometry Nodes"
4. In the modifier choose "Mystique Scales Effect" from the dropdown
5. Add an empty to your scene and set it as the "Control Empty" in the geo nodes modifier
6. Add a material to your object with 2 Shaders. These are your 2 Materials you can set up like you normally would
7. In the shader editor go to "Add" --> "Group" --> "Mystique Effect Shader Control"
8. Plug both shaders into the new node group and the output of the group into the "Surface" input of the "Material Output" node

If you have multiple materials on your object, repeat step 6-8 for every one of them.