usually for hands I never map it out. I more think of it as shapes like a ball-jointed doll of sorts.
Your palm(I use this to refer to the main part of your hand). It isn't the same width all throughout. It starts at the width of the wrist and gets wider as it goes to your fingers. The fingers are cylinders rather than lines like you would use when mapping it out. If you use lines it can confuse you.
The middle finger is the same length as the palm. I usually start with the pinkie and index finger. The ring finger is usually similar to the index finger.
The thumb starts at the base of the palm. It sticks out.