1. Open a textured image such as a scanned-in piece of cardboard in Photoshop
2. Convert the texture to black and white by going to Image > Adjustments > Black and White
3. Adjust the contrast by going to Image > Adjustments > Levels. A higher contrast image will yield a more defined texture
4. Save an image as a PSD by going to File > Save As and selecting the .PSD option
5. Open the PSD in Illustrator
6. Convert the texture image to vector shapes using the Image Trace tool (Window > Image Trace)
7. Select a Preset that fits your texture. For scanned-in paper textures, the Shades of Gray preset works best. Trace by clicking the "Trace Button"
8. With the image selected, go to Object > Expand and ensure both "Object" and "Fill" are checked
9. Save the texture as a .AI file