Step 4: Tie off the ends
Take your zip ties and cinch both ends of the sock to make sure nothing escapes. If you used activated charcoal or the aluminum silicate, you might want to consider a second sock to prevent any dust from filtering through.
Step 5: Stash your fancy new air freshener under the passenger seat
Search around for the best place under the front passenger seat to put your sock. Why the passenger seat? Just in case it comes loose, you'd rather it not go under one of your pedals while you're driving. Ideally, you want to make sure it's not blocking an air vent (most cars have one that exits under the seat that you've likely never seen) or sitting on top of a speaker (some cars have subwoofers under the seat). You also want to avoid the motor and/or track on which the seat moves, and you don't want to be stuck against any heating elements. Basically, be careful, and you'll be fine.
That's it. You're done! Now, order some really exotic takeout with confidence.
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Aaron Miller is the Cars editor for Thrillist, and can be found on Twitter. He once tested the volumetric efficiency of air conditioners based on how quickly they removed the smell of pad Thai.