Do the same process with the other images in the Image List Panel. You don't need to be precise in marking the objects. They only have to be more or less covered by the brush strokes.
As you go to the other images, you will notice that SmartRemove allows you to see the brushed-in objects and positions from the other images to help you avoid overlaps of your elements.

Tips:
- For better results, select one image with an element that is on the foreground of the other elements you want to add and lock it by clicking the Set as Key Image
button.
- You can change the color of the Brush in paint by clicking the paint color box.