Answer:
He forges letters by the citizens and plants them for Brutus to find.
Step-by-step explanation:
Cassius did that in order to remove the doubts on Brutus´s mind to start the rebellion against Caesar. He did that not because he loved Rome as he convinced Brutus. He did it because he was jealous about Caesar´s power and believed that he was not suitable for the position of emperor.
I hope this answer helps you.