For this you will need a computer, word processor and a printer (or a typewriter in lieu of all three), stationary and manila envelopes. Staples only had #12 envelopes on the shelf, and I highly recommend #10 instead - I could not force my computer or printers (two different brands) to print #12 envelopes properly! This must be an anomaly with Microsoft Office 2007.
First type up a formal phony letter from the I.R.S. Audit Division (I used cotton rag paper with a watermark to make it more 'official'), then type a return address like, "U.S. Department of Revenue, Federal Courthouse, blah blah" and address it to your Victim. I fell for this one myself and had the envelope laying on the nightstand for two weeks before I had the nerve to open it!
Disclaimer: I have no idea if this is 'legal' or not, or if it violates USPS rules and policy, etc. This is intended for fun only and should not be used on persons with weak hearts, your enemies, curmudgeons, etc. A little imagination and you can tailor it to individual circumstances (Dept. of Public Health, H.I.V. Testing Facility...Kleegle and Lawson, LLC, Divorce Attorneys...etc.).