First things first, a metaphor says that one thing literally is another. Some examples you may have heard before... 'raining cats and dogs', 'shooting the messenger', 'the Good Shepherd'. Even Shakespeare had a crack at it: 'But soft! What light through yonder window breaks? It is the east, and Juliet is the sun.' (Romeo and Juliet Act 2 Scene 2).
An extended metaphor does the same thing, but the picture is built up gradually throughout a longer passage. Examiners like them because they help make your writing more sophisticated and show you've thought about the overall message.
Extended metaphors can be pretty daunting but they also quite fun once you are confident writing them. Here's how I remember the basic rules:
Is the metaphor relevant?
In other words, does the image you're using to describe your subject (the place, person, situation, object) help to convey your meaning. For example, if you're describing a battlefield, make sure you pick an image that is suitably solemn, respectful or harrowing. If you can, use your metaphor to add some extra meaning to your description.
Is the metaphor accessible?
Whilst you might be a specialist in korean cooking, American football or ice dancing, your examiner might not be. Make sure you pick your extended metaphor carefully and that the meaning can be worked out by almost everyone.
Is your metaphor proportionate?
If you're trying to emphasise the scale of an event, compare it to something bigger. Likewise, if you want to get cross how small or insignificant something is, use images like a fruit flie, a one pence coin, or a grain of sand. (Once you get the hang of this you can invert it to create pathos, the feeling of being underwhelmed.)
The building blocks (The lexical field)
If you want to say that an old woman is wise, you might pick the traditional symbol of wisdom - an owl - to compare her too. You might say that she has 'amber eyes', 'feathery' hair or clothes, 'talons' for nails etc.
If you want to describe a particularly loud or flamboyant person who draws lots of attention you might start subtlely referring to their surroundings as a 'theatre', their friends might be their 'audience', the chatter following what they say might be 'applause'.
Importantly, try out lots of different things in your metaphor. You only need to use one continuing metaphor in a piece of creative writing. Once you've mastered it you will find that extended metaphors creep into all sorts of writing, because they are a great way to get across a complicated message.