Collect your photos, both the photo you will be adding the streamers to and a photo of falling streamers. Be sure to take or choose a photo of falling streamers with a solid background color and with no other subjects in the frame. A stock type photo of falling streamers is ideal. If the photos are not already in a digital format, use a scanner to transfer them to your computer.
Open your photo editing software and import the two photos. Depending on the program, you may need to unlock the layers the photos have been imported on. You will be working on the photo of the streamers first.
Select your program's auto select tool (in some programs known as the wand tool) and click anywhere in the solid background behind the streamers. If there are areas left out of the selection, switch the auto select tool to the "add to selection" mode and then click on the unselected areas. If the edges aren't as close to the streamers or as smooth as you'd like them, enter the edit selection menu (sometimes "refine selection") and expand the selection slightly.
Clear the background by using the clear option, delete selection, background eraser tool, a mask or the eraser tool. When the streamers are isolated, use the edit menu to "select all" and then copy the photo.
Switch to the photo you will be adding the streamers to. Use the edit menu to "paste." The isolated streamers should appear, though they may not be the same size as the photo you're adding them to. If this is the case, use the cursor tool to resize the streamers until the two photos match in size.
Use the zoom tool to zoom into any jagged or unnatural edges surrounding the cutout streamers. Use the eraser tool on a small setting to clean up these edges.
Use the contrast, levels or lightness tools (or all three) to adjust the appearance of the streamers. Tweak these setting until the streamers seem as though they are under similar lighting conditions as the photo.