1. Scan the image or save the photos from a digital camera or ascertain a digital image of the source for the texture you want to work with in some way.
Textures can also be made without a source image to start from. You can simply paint something in Photoshop which looks like what you want for a texture.
Another neat trick in Photoshop is to use the the Filter > Render > Clouds and Filter > Render > Cloud Difference filters as a starting point for your textures, to which you can apply various other filters to achieve the look you want.
2. Select an approximately square portion of the image at least a couple hundred pixels square that includes what you want to texture and as little else as possible.
3. Straighten the image if it is twisted (using Image > Rotate...). Use the Image > Effects options of Skew, Perspective, and Distort to make the image as close to an orthagonal view (plan or elevation) as possible.
4. Crop the image so it is square in shape and includes only the part of the image you want to create a texture from. By default the size of the texture will render as two feet square in DesignWorkshop, you will need to edit the DW Material Prefs file if you want your texture to tile at a different size.
5. Change the image size to 256 pixels by 256 pixels (you may need to turn off Proportion constraints). Copy the image and paste it in Desktop Patterns to see how well it tiles before proceeding. If it is acceptable you can go to step 9 now.
6. Select "Filter > Other > Offset..." Set the offset to about 75 (pixels) in both dimensions and make sure "Undefined Areas" are set to "Wrap Around". You now have an image that will tile perfectly at the edges and you can clearly see where the seams are and work with them easily (be careful not to edit the edges of the image as this will mess up the tiling).
7. To eliminate the seams try using the rubber stamp tool to copy the image from a good portion of the image to cover over the seams (this works well with fairly random textures suchas stucco or carpet). Also try using the lasso select tool to copy portions of the image that is not on a seam to cover seams (this works well on images that have defined non-orthagonal forms, suchas flowers or rocks), or the marquee tool and then copying parts of the image to cover seams (this works well on orthagonal textures such as bricks or shingles.
8. Use the above tools to cover noticeable anomolies in the image, because these will likely form an annoying pattern when the image is tiled repeatedly.
9. Copy the image and paste it in Desktop Patterns. If it looks OK save it as a PICT file and add it to the Textures folder. If not return to step 6 and continue making corrections.
10. After it looks good in Desktop Patterns and you saved the PICT to the Textures folder, try it in DW and if you don't like it return to step 6 to correct problems with the image itself and edit the DW Material Prefs file to correct how it renders in DesignWorkshop.