Before the exam:At A-level, you are told that you will have to translate an unseen passage from a specified text. Read as much of the text as you can, to get a feel for the author and the plot of the text itself. If you know, for instance, that in Ovid's Metamorphoses, that someone always gets turned into something else, you will be prepared for such a passage and have at least a vague idea of the stylistic imagery and structures that are entailed. You will also have a better understanding of the writer's style: e.g. whether they're prone to using complex structures like asyndetons and tricolons, or whether they prefer more straightforward language.
During the exam:The first thing to do when you start the exam is to keep calm. Don't rush into translating immediately: instead read all the available information to understand the context as much as possible. First read the English blurb at the top, as this will give you an idea of what exactly is happening in the text and the kind of scenario that you can expect. Second, scan the passage for unfamiliar vocab - anything you don't recognise, highlight it and check the given vocabulary at the bottom of the page to see if it's provided there. Then separate the passage into sections: divide the sentences up into clauses and sub-clauses. Parse all of the words, identifying their conjugation or declension, and identify grammatical structures like ad clauses or ablative absolutes. Make a rough translation of the text, using what you know of the vocab and grammar to translate the clauses and sentences. Read over your translation. When you come across the unfamiliar words/clauses that you highlighted earlier, see if you can contextualise them in the background of the other words that you do know. Examiners give more marks for grammar comprehension than vocabulary, so make sure that you clearly demonstrate your understanding and grasp of all the words. Write out the translation as your final version. Make sure the English flows well and makes sense, double checking any structures or words that you're unsure of.