Locate the fan or fans for incoming air where they will pick up the least amount of dust. Could be high or low depending on your set up. In the booth I'm using this weekend (which is smaller than the one in the pictures) I'm using one fan on high to supply air and another fan on the opposite side on medium to pull air out. Both are sitting on the floor.
All you need is enough pressure inside to keep the plastic walls from moving freely. The side benefit is that it also keeps the dust down. You'll never get rid of all the dust painting in your garage but if you plan to color sand and buff, any dust nibs you get will sand out anyway.