Instructions:
1. Open the file in photoshop and locate the Layers panel.
2. Find the layer highlighted in red, and double click the thumbnail of that layer.
3. A new window will open displaying the placeholder design. Add your design, filling the entire area. 
4. File  Save (or cmd/ctrl+s) and close this window. Back in the main file, your design should now be on the paper!

Changing the background color:
1. To change the background color, go back to the Layers panel and open the folder titled "Colored BG (editable)"
2. Select the layer highlighted in green, and open the Properties panel. Adjust the Hue and Saturation sliders until you have the color that you want. (mid-range colors work best)
3. If you just want a white background, go back to the "Colored BG (editable)" folder and click the little eye symbol to hide the layer, revealing a light background.

To make each page editable independent of the others:
1. Right click on the red-highlighted layer and select "New Smart Object via Copy", it will create a duplicate of the first page in the stack. 
2. You can then edit that first page separately from the rest of the stack of papers by double clicking on the thumbnail of the new layer you created.
3. (The shadow under the paper will become extra dark because of the two layers on top of each other, so hide the original version of the layer that you copied to fix that.)

4. To change the other pages also, open the "stack of paper" folder.
5. Right click on "paper 2", "paper 3", and "paper 4", or whichever of those you'd like to change. Once again select "New Smart Object via Copy. 
6. You can edit the new smart objects by double clicking on the thumbnail of the new layers that you created. 
7. (And once again hide the originals, to maintain the maintain the correct darkness of the shadows)