Your sentence is almost correct, but it can be improved for clarity and grammar. Here's a better version:
"Draw a swarm of tiny krill, showing some of them enlarged through a magnifying glass."
Explanation:
- Use a comma to separate the two related parts of the sentence.
- "Showing" is a better verb here to describe the action of displaying some krill larger.
- "Enlarged" fits better than "large" when talking about something seen through a magnifying glass.
So, the corrected instruction is clear and easy to understand.