Click once on the editing canvas or click the + button, you'll see a menu that allows you to add scraps.
To editing the scraps you add, you can either double click or right-click on it to open editing menu.
If several photos are overlap each other, clicking once on a photo will bring the photo to the top. There's also the magic dot at the lower right of the photo. You can use the magic dot to rotate and resize a photo at the same time.