To measure your sit bone width for a proper bike saddle fit: (i) Place a piece of cardboard or a specialized sit bone measuring device on a firm surface. (ii) Sit on the cardboard or device, ensuring your weight is evenly distributed. (iii) Stand up and measure the distance between the center points of the two indentations left by your sit bones. (iv) This measurement corresponds to your sit bone width and helps in selecting a bike saddle that provides proper support and comfort.