Slightly different from those above...I used inexpensive plastic tarps from Fleet Farm. They are heavier weight than visqueen and don't tend to get sucked inward or move outward with the air pressure from the fans and the gun. They also come with grommets so they can be easily hung from the ceiling on hooks which can be left in place for the next time you need to have a paint booth. I also find that they do not require being weighted around the bottom edge. Note also in the picture the Jet air cleaner hanging from the ceiling. In addition to my intake and exhaust fans (I use box fans), I run the Jet on high to help filter the air during the shoot.
In this second shot you can see my "mixing table" in the background as well as a sheet metal box on the table which is for testing the gun pattern and making final adjustments to the paint flow before actually shooting the car. Also note the blue tarp taped to the floor which was an extra precaution because the car body was so close to the floor I didn't want to have any dirt from down in the surface of the cement blowing up into the paint. In addition, I opt to spray the floor down with a little water mist before shooting each coat. I use a small garden type insecticide sprayer to lay down the water. (Note: there are many who argue against using this technique, so keep that in mind.)
Final note regarding "stupid things we all have done". Don't EVER throw open your overheard garage door immediately after shooting. Let the paint dry first. Many a guy has opened the door only to have a cloud of dust and debris fall from the rails down onto a pristine paint job.