In words. Maybe get a piece of paper and a pencil....think of the dart shown, two edges, equal length, coming together at a point. We can easily measure the endpoints of the lines. The line that cuts the long edge is placed in such a way that when both such lines are placed the distance between those endpoints is the width of the spread. The endpoint that intersects the shorter edge does so 3/8" from the top edge. So, for a given dart, fold the fabric over so that the endpoint on the short edge overlays the endpoint on the long edge. Now if you take a ruler and lay it out on the doubled fabric so that the ruler edge passes through the overlaid points and extends up and crosses the fold, note that any place we place that ruler, if we were to cut along that line we would create a dart with exactly equal side lengths. By experimenting I've found that 17" length is good for the head, and 14" is good for the foot.
Now I don't really cut along that line. Instead I draw a line on the fabric, pin the two sides together, and run a stitch along that line. THEN I cut, just offset from the line a little.