What is the best way to approach an unseen translation?

The most important thing is not to dive in right away. It can be so tempting, especially under exam timed conditions, but it will make things much harder. Start by reading all information given about the translation - the english introduction and any vocab lists given - this helps to give an idea of the text context. Then read all the way through the Latin twice. Try to translate sections in your head and begin to get a feel for the flow of the text. With this in mind, then start at the beginning of the text, working sentence (or clause) by sentence. Keep in mind the style of the text - poetry often works by enclosing meanings in couplets, whereas prose does per sentence. Annotate the text by underlining main verbs, highlighting different cases in nouns etc to stay mindful of how the grammar works in the sentence. This will often provide more marks than vocab - so don't panic if there are words you don't know, this is natural. Work slowly and methodically in this way through the text, before making sure you read through your final translation several times, allowing time to adapt the translation to fine tune your style.