I've noticed a lot of achievement guides for the game where written when it first came out and are 1. a little sloppily written and 2. not necessarily applicable to the game as it currently exists.

Here is how you unlock It's Raining Cars in one, easily-to-do move.

1: Equip the broom.

2: Stand on the highway until a car is in front of you.

3: Look directly at the ground

4: Step forward so your player model is touching the front bumper.

5: Look up as fast as your sensitivity/controller/mouse will allow.

The tip of the broom is a physics object and interacts with cars. Old methods involving 'sweeping' cars by clicking sweep while running at cars is unnecessary and adds an extra layer of unpredictability. Just flip upwards while the broom is underneath the car and it'll go flying. I got it in one try with a small hatchback.