How to draw Internal structure of mammalian Heart

The heart is mesodermal in origin. It is a thick walled, muscular and pulsating organ, situated in the mediastinum, and with its apex slightly turned to the left, it is of the size of a clinched fist. The heart is covered by a double walled pericardium which consists of the outer fibrous pericardium and inner serous pericardium. The serous pericardium is double layered, formed of an outer parietal layer and an inner visceral layer. The parietal layer is fused with the fibrous pericardium, whereas the visceral layer adheres to the surface of the heart and forms epicardium. .The two layers are separated by a narrow pericardial space, which is filled with the pericardial fluid.

1.First draw basic heart shape on the paper with its narrow end towards your Right .

2.Then draw Precaval vein and Post caval vein on the
Right side of the heart.Actually your Left becomes Right for the heart here.

3.Make the Muscular wall of the heart, ensuring the Left ventricle has thicker wall than Right ventricle.

4.Make Pulmonary aorta emerging from Right Ventricle.

5.Then draw Systemic arch that emerges out of Left ventricle. Make three nubs on the arch.

6. Now draw the valves inside the Systemic aorta and Pulmonary Aorta. 

A specialized cardiac musculature called the nodal tissue is also distributed in the heart. A patch of this tissue called the sinoatrial node (SAN)is present in the right upper corner of the right atrium near the openings of the superior venae cavae. Another mass of this tissue, called the antrioventricular node (AVN) is seen in the lower left corner of the right atrium close to the atrioventricular septum. A bundle of nodal fibres, called atrioventricular bundle (AV bundle/ His bundle) continues from the AVN into the inter-ventricular septum. It divides into right and left bundle branches. These branches give rise to minute fibres called the Purkinje fibres that extend throughout the ventricular musculature of the respective sides. 

SAN consists of specialized cardiomyocytes. It has the ability to generate action potentials without any external stimuli,hence called "Pace maker". AV node is a relay point that relays the action  potentials received from the SA node to the ventricular musculature. SAN can generate action potentials every 0.6 sec.Autonomous and hormonal coordination systems take charge of increasing or decreasing the rates, depending on the situation.

7.Now replace the smooth inner walls of ventricles with ridged structures Columnae carneae

8.Now draw Papillary muscles emerging out of Columnae carneae towards the valves. We are done my friend.