The figure given in NCERT book is one of the simplest figures of human heart possible. This is because the structure of human heart is quite complex. But you will find the figure simple if you first understand the theory behind the figure. Once that is clear, you will find the diagram easy. But drawing a neat figure comes only with constant practice.
You can also see the diagram given in the following site. But remember that all the labeling given in the NCERT figure should be shown.
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