The pen tool is weird at the beginning, but unbeatable when you want to get clean, smooth edges around objects. Sometimes, the marquee based selection tools will produce a too detailed and messy result.

1. Download the below image in full resolution.
2. Open in Photoshop
3. Use the pen tool to create a path around the sheets.
4. In the options field, use Make: Selection. 
5. Then click “Select and mask” in the options field.
6. Use “Output: New layer with layer mask”. 
7. If you want, you can edit the mask further by using the black/white brush.
8. Save the file as PSD (or PNG), which support transparency. You can now use this file in InDesign in a layout.