@g_whin ok, in my exam, i mostly got describe graphs, my steps in describing are:
Tell the title of the graph.
Describe the x-axis, then the y-axis.
Explain the key points in the graph, as you observe it. Like telling where is the minimum or maximum values, which has the smallest or greatest, etc.
If you still have some seconds left, just talk and explain whatever you see in the image or graph, until you reach 40secs.
For retell, my first sentence should be telling the topic, and summary of the whole lecture. And supporting lines were from keynotes i wrote while listening.
You should write the keywords, so you will have enough to say until you reach 40secs.
For both, i always made sure to have correct grammar, correct pronunciation, use synonyms instead of repeating to say again key words.