I did this once to a new co-worker that just moved into the area. I made a fake envelope and letter with the City seal and postage cancellation, and stuck it in with the rest of his mail.
The letter I "sent" him said that his pet snakes were in violation of a city ordinance against exotic animals. He was informed that he had 24 hours to get rid of them. He was furious, and ready to fight.
One could do this stating they have:
A pet that needs to be removed .
There car has too many tickets and is about to be impounded.
A warrant for their arrest.
Are about to be Audited or owe back taxes.
That they've won something
Anything that they know they are in the right about, or feel strongly about, that will get their emotions going. Or that will make them very nervous.
Step 1: Get supplies
First thing you need is a letter that has come through the mail lately with a permit stamp or metered mail postage. City and state org. and corporations don't use stamps. The second thing is to go online and find a Logo for the agency you are pretending to be. You could scan one off of a letter if you already have one. These 2 things really set up the envelope to look official and help support the joke. You will need a scanner (or camera if you don't have a scanner) and printer.
Step 2: Create the envelope
Scan off the permit/postage cancellation and save it as a picture. Then open Word or some other program that will let you address and work with envelope sized paper. Then put your picture of the postage and Agency Logo in the corners and type in the Address of your mark. To give it some even more authenticity, try to use the postage bar code across the bottom. Print this out and see how authentic it looks.