Tiled Background

I made this "corrugated" background in basically four steps:

1. Use the Fill tool to make a blue to blue-black linear gradient at 90 degrees.
2. Copy the image with File | Copy.
3. Enlarge the canvas of the original image to twice its width.
4. Paste in the copied image with File | Paste as Selection, and mirror the copy.
5. Overlap the original and mirrored images so that the dark "depression" is not too wide.

Afterward, I used the handmade seamless pattern method to refine the edges to produce a smoothly tiling image.