The heart diagram in NCERT is not difficult if you understand the structure of heart and its associated blood vessels.
First draw the 4 chambers of the heart. Then show the opening of each atria to its corresponding ventricle. Then draw the blood vessels arising from the associated chamber. Example Vena cava bringing blood to the right atrium and pulmonary veins bringing blood to the left atrium. Understand the theory first, then the diagram becomes very easy to draw.