For ventilation (like in a garage) there are four things to do:
1) Get all cars out so the paint doesn't stink up the fabric like it does)
2) Open the garage door
3) Wear a mask (for your own safety, I got light headed when I didn't use one)
4) Paint near the edge of the garage (near the door) so you the area gets ventilated quickly
Hmmm, also, if the wind blows things into your arage, it is a good idea to have a fan blowing the air out of the garage, but that is optional.
I prefer using the backyard. If it is not going to have good weather, simply don't do the mod that day.