The focus on these types of questions is on the STRUCTURE! And that's something that really needs to be perfected.
Two books I studied were Jane Eyre and the Great Gatsby, so this answer will be centred around that.
Initially, finding key examples of symbols/motifs in both, when planning, so:Gatsby: the colour green, symbolism of the eyes, Valley of the Ashes Jane: the colour red, motif of the supernatural, the moon
Such motifs would be developed through finding examples of them in the text (and making a revision table of different devices and examples). Then finding how these are effective for both the texts, before comparing them mainly in the second half of the essay. Also focusing on a strong PEEDL structure and doing timed essays as practice.