I made a new accessory mesh, cloned from the basegame rimless glasses, however now the accessory which is supposed to be made out of leather and fabric has a metallic shine all over it.

I looked at tutorials, which mostly consist of how to add shine, hoping to work my way backwards from there. However, I have not been successful as most of the actions described in the tutorial don't seem to be applied to the package.

Am I missing something?

You need to lower the reflectivity in the TXMT. Make sure to hit Commit before you hit Save.

That's the point, the reflectivity is already at zero by default. I'll attach the file, in case someone wants to take a look at it.