You need to learn each of the diagrams and memorise them with a proper sequence of labelling.
The trick to memorising the diagrams is to understand the theory and then connect it to the diagrams.
Practice the image and labeling every day. This helps you for a better recollection of the diagram.
Prepare a flowchart of all the diagrams and use coloured pens/markers to highlight the labels in the diagram. This method catches the attention & helps in memorising them better.
Refer to NCERT books preferably to learn diagrams; this can be very useful in memorising them.