The paired statement technique
Each point in a compare answer should refer to both items in the same sentence:
Correct: "Arteries have thick muscular walls, whereas veins have thinner walls."
Incorrect: "Arteries have thick walls." (then separately) "Veins have thin walls.": this describes each separately and may lose a mark even though both facts are correct.
Use comparative connectives: whereas, but, while, however, in contrast, similarly, both.
Include both similarities and differences
Unless the question specifically asks for "differences only", a compare question expects both similarities and differences. Start with one or two similarities, then move to the differences. This shows a deeper understanding.
Example comparing mitosis and meiosis: "Both involve division of the nucleus. However, mitosis produces two diploid cells, whereas meiosis produces four haploid cells."
Organising a compare answer
Choose key features and compare them one at a time. Do not describe everything about item A and then everything about item B. A table format in your planning (even if not in the final answer) helps ensure you address the same feature for both items.