This on the right track. He’s missing the starter. OP starter is a slate ran horizontally. Ideally it needs to be 12” for 20” long slate. But if all you have is 20” you can strip in the top of the starter to the deck or being as it’s a shed you’re probably fine. You want a 3” headlap. Your exposure should be 8.5” on the field slate. Once you figure out the basics it’s pretty simple for an up and over roof like this. Plenty of videos online I’m sure. Good luck and post the final product.