Thanks for the tutorial, I was looking for just something like this!

I have a question though. I've tried this out in Gimp 2.10 and the grid highlight layer seems to cover the slate background with its white color and creates a grayish, flat background. I'm quite new to GIMP and can't figure out how to get the slate background to show through like in the tutorial images.

Anyone have any suggestions of what I might be doing wrong?