You make a layer background of what you want the scraper underneath to be (rainbow, gold, silver, blue etc.) and then, on a separate layer, colour the entire canvas black. Make sure that layer is above the coloured. Then, with the eraser tool, draw on the black. The colour will show through. c: Hope that helped!
Anyone can make a scratchboard, and seeing as you're not providing lines or a background, it's not a suitable editable. I'm sorry, but this shall be locked.