Click once on the editing canvas or click the '+' button, you'll see a menu that allows you to add scraps.
To edit the elements you add, you can either double click or right-click on it to open editing menu.
If several photos are overlapping, you can click once on a photo to bring it 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.