The creative writing section is some people's favourite part of the exam, and some people's most detested. The key thing to remember is that, above all, the examiners are looking for descriptive language- similes, metaphors, adjectives, sibilance, alliteration etc. But be sure to avoid the cliché ones such as 'white as snow' or 'glittering like a diamond.'
When you see the question, e.g. 'Describe a childhood memory', although no one knows if it actually happened or not, in my experience it is a lot easier to describe something that did happen, even if you tweak bits to make them more dramatic/ poignant. Once you have the basic plan then add in descriptive language, thinking of comparisons which evoke all five senses.
Definitely avoid using a lot of dialogue or it can become tedious to read. If you absolutely cannot avoid it then opt for reported speech over direct speech.