Writing reports are not necessary to excel in the Law Enforcement career and reach the top. If you want to do this though it's simple but a little time consuming.

Step 1.
Become friendly with a Sim until the interaction 'Question' appears.

Step 2.
'Question' the Sim.

Step 3.
Go home to the computer and 'Write a Report'. It will give you options to write reports the Sims you have questioned.

And that's it. Simple.