Drawing the four chambers
Draw a rounded shape divided into four sections. The two upper chambers are the atria (singular: atrium): the right atrium on the left side of the diagram and the left atrium on the right side (because the diagram shows the heart as if you are looking at the patient). The two lower chambers are the ventricles. The left ventricle wall must be drawn thicker than the right ventricle wall because it pumps blood at higher pressure to the whole body.
Adding valves and vessels
Draw atrioventricular valves between each atrium and ventricle: these prevent backflow from ventricles to atria. Draw semi-lunar valves at the base of the aorta and pulmonary artery: these prevent backflow into the ventricles.
Attach the vena cava (two large veins) to the right atrium, the pulmonary artery leaving the right ventricle, the pulmonary veins entering the left atrium, and the aorta leaving the left ventricle. Draw the coronary arteries on the surface of the heart muscle.
The route of blood
Deoxygenated blood returns from the body via the vena cava into the right atrium, passes through the right atrioventricular valve into the right ventricle, and is pumped through the pulmonary artery to the lungs. Oxygenated blood returns from the lungs via the pulmonary veins into the left atrium, passes through the left atrioventricular valve into the left ventricle, and is pumped through the aorta to the body.
This is a double circulatory system: blood passes through the heart twice in one complete circuit.
Coronary heart disease
The coronary arteries supply the heart muscle with oxygenated blood. If they become blocked by fatty deposits (atherosclerosis), the heart muscle receives less oxygen and may be damaged, causing a heart attack. Label the coronary arteries clearly: they are on the outer surface of the heart, not inside the chambers.