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Then make sure you understand the grammar that might be relevant (for example, for a french test on holidays I might make sure I know the past tense to talk about previous holidays, adjectives to describe the area, comparisons to compare previous holidays, opinion words etc.)
Also look at previous assessments or marked work to see where your weaker areas are (e.g. for me, I often misuse the subjunctive in french and misspell plurals in German) and do some individual practice on that.
You can use websites such as languagesonline to build up or consolidate your knowledge.