Instructions
Download the DDS Plugin for your photo software from the official website. The two main plug-ins are available for GIMP and Adobe Photoshop. Follow the installation instructions for your specific software.
Open a new file, and set the dimensions to 1024 by 1024 pixels. If you want, import a texture file from "The Sims 3" game, so you can have a starting point and then adjust it yourself. The texture files are located in the "Source Textures" folder of "The Sims 3" data.
Use the photo tools to create or edit your texture. The paintbrush can create all kinds of designs to use. Apply different opacity percentages to blend colors together.
Add text or images on the texture, if you want. Depending on how you use the texture, the image or text could become distorted when implemented into the game.
Go to "File," "Save As..." and select the "Source Textures" as your save location. Add ".dds" for the file extension, and click the "Generate Mip Maps" box to continue. Change the compression to "BC3/DXT5" and click "Save" to complete the texture.