This is going to sound very basic, but before you start working on a report, have a sample and study it. Don't just look at it; really study it. Ask yourself these questions before you start.
How does the department write case numbers? Is there a hyphen in the case number or not? How do they write the information in the header? Is it all-capped or upper- and lowercase? How do they write their headings? Again, are they all-capped or upper- and lowercase? Are the headings bolded or not? Do they have a colon or not?
How are the paragraphs set up? Are they indented or not? Is there a blank line between the paragraphs or not? Is the text justified, meaning it's flush with the margins on both sides, or is the right edge jagged?
How do they write the date? Do they uses dashes (7-1-15) or slashes (7/1/15)?
How do they write name in their reports? Do they all-cap names or write them in upper- and lowercase? If the department all-caps names, do they all cap everybody in the report or just the names of the people the report is about?
How do they label their victims, suspects, and witnesses? Do they use V-, or VIC-, or do they spell out the words?
How is the officer's name written at the end of the report? Is it all-capped or upper- and lowercase? Does his ID number follow his name?
Finally, research the law enforcement agency's guidelines and find out how the department cleans up the report. Do they delete unused headings, or do they fill in unused categories with "None" or some other word?
Once you have the answers to these questions and have a firm understanding of how to format your report, then you are ready to start transcribing the officer's words.