The advice so far has been pretty good, but here is a different take on it, anonymous tip. You get the best of both words, but be careful to really be anonymous
You have 4 choices:
1) telephone - if you can afford a disposable cell, or you know of a payphone that still works ...
2) email - be sure to create a one-time use account for this purpose, use a public computer ...
3) note - don't handwrite the note, don't use your own printer/typewriter, wear gloves, don't lick the envelope...
4) psychic - hire a psychic to beam a message, make sure to hide your face, on second thought, the psychic could identify you no matter what you do, too risky

whatever you do, don't include your name, or your address, or your license plate, or social security number...