After having some email chats with various friends over the last little while on this topic, I figured I would write this quick tut up just as a helpful idea for your digi-stamping arsenal...
1. Before you begin, decide whether you want to color in the image digitally (see my tut here). If you do elect to color your images do that first and save each file with a new file name (e.g., Tree-Colored) as a JPEG. Close all your images and re-open the saved JPEG files. In this example, I will work with blank images.
2. Working with your first image, ensure the layers palette (F7) is open.
3. Right-click the background layer.
4. Click Duplicate Layer.
5. When the Duplicate Layer dialog box displays, click OK.
6. In the Layers palette, select the duplicate layer.
7. Select the Magic Wand tool. (This tool is displayed in the previous image.)
8. Click outside the out-most edge of your image as shown in this example and press Delete.
9. Select the Magic Wand tool again to turn it off.
10. Repeat steps 2 - 9 with the rest of the images you want to mix to create a scene.
11. Ensure all the images are open and visible in your window (if necessary, click Window > Arrange > Float All in Windows).
12. In this instance I am going to place the Amber Girl next to the tree (which would be impossible if all the white space were still there!). From the Layers palette, ensure the duplicate layer is selected.
13. Drag the duplicate layer over to the tree image.
14. Since the Amber Girl is a little big, I am going to free transform her using the handles at the edge (if you want an even scale of the image, ensure that you press Shift as you do this).
16. When prompted if you want to apply the transformation, click Apply.
17. My Amber girl is then placed and sized as I want her. See how she is tip-toeing on the tree skirt with no overlapping lines? Now that is masking made easy! I then added my presents to the base of the tree using steps 12 to 16.
18. Here is my finished image ready to print off and color! Voila!