Select two  job posting off usajobs or usagov that are a higher position in the Criminal Justice field.
Copy each posting and paste them into page one of a Word document.
Choose one of these two openings to use as the basis of your cover letter.

on page 2 compose your cover letter.
Address the letter as you would if you were applying for the specific job you have chosen.
Save your postings and cover letter as one Microsoft Word document.  Follow appropriate formatting requirements for your cover letter.