In Photoshop, drag and drop an overlay over your document.
Scale and transform the layer so that it fits your image then press enter to apply the transformation.
Change the blending mode to "Linear Light" or "Hard Light".
OR
You can also use the included Photoshop pattern file. Add a new Pattern fill layer (Layer > New Fill Layer > Pattern).
Open the pattern picker.
On the top-right corner of the pattern picker, click on the panel menu. Select "Load Patterns".
Browse for the "Folded Paper Textures.pat" file then click OK.
Pick any folded paper pattern then adjust the scale setting so that it fits your document.
Click OK to exit out of the pattern layer settings.
Change the blending mode to "Linear Light" or "Hard Light".
Double-click on the pattern layer thumbnail to edit the settings again. Pick any folded texture that you like to use.
Note: The Photoshop patterns make it easier to preview the different textures but they are lower resolution. For the best image quality, use the JPEG images. You can hover your mouse over the patterns in the pattern picker to find the filename.