Palm is a pentagon. Sides are roughly the same length, depending on the person.
Base of pentagon is the wrist.
The next side of the pentagon, on the inside, leads to the thumb. The thumb is the same length, overall, as that side.
The apex of the pentagon is where the index finger come from. It generally points straight away from the center. Its tendon goes straight up from the middle of the wrist to the apex. The finger is roughly the same length as that tendon.
The pinky finger extends out from the outer side of the pentagon, and is about the same length as that side.
The middle and ring fingers are about the same length as the index finger, and are evenly spaced between it and the pinky.
A natural relaxed hand has the fingers curved in slightly.
Sketch these basic shapes lightly, then fuck around with them until they look okay.