This technique can easily add alot of depth to your images if you are willing to put in the time and effort it takes to complete each layer.
1.Consider your entire image. Where will you need flat space for painting and where is it fine to create uneven surfaces. Maybe you use the raised surfaces as your painting surface.
2. Locate the various items you will be pacing onto your canvas and roughly lay them out.
3. Begin with applying a 1st layer of paint and let it dry
3. begin gluing down your objects. Consider height of each piece and weight.
4. paint again. Don't be afraid to wipe it off to create a more washed out look or possibly sand it down once it is dry for a more distressed look.
5. repeat until you achieve the desired effect