Find shoe and boot spats patterns on ActionAGoGo.com, TheCraftyAsian.com and Simplicity.com. The patterns include instructions on how to measure a specific set of footwear and cut fabric in order to create custom fits.
Simplicity.com offers a detailed set of instructions for creating custom spats. The materials required are several different types of fabric, which are used to create the foundation of the spats, adornments, and tools like scissors, sewing needles, straight pins and a sewing machine. It also offers links to spats patterns for both children's and adult shoes. Following this site's instructions, you can measure and cut out fabric following the pattern and create a wrap-around spat with optional buttons.
TheCraftyAsian.com contains a tutorial on how to make a spats pattern tailored to a specific set of shoes. Doing so involves taking two pieces of scrap fabric that are slightly taller than the shoes and wide enough to be clipped together in the back. This requires clipping the fabric together at the back of the shoes and pinning the front as close to the shoes as possible. You can add custom embellishments to this existing pattern, or you can tailor the pattern itself to a different pair of shoes.