Answer:
We're assuming that the question refers to a speed radar gun, and not to
a Doppler weather radar, FAA air-route surveillance radar, or military radar.
Get a microwave source that runs on 13 volts and radiates at 10.525 GHz.
Connect it to your car's electrical system and mount it behind the front grill.
Speed radars will not be able to get a reading from the front, and they'll be
effectively jammed. The operator will stop you to see what you've got going.
Hopefully, he'll ticket you for interfering with local traffic regulation, and then
send your ID to the FCC, who will fine you for operating an unlicensed RF
transmitting source. And that's how you do it.