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When you sand drywall, you are going to get powder everywhere. If you've ever had a flour accident in the kitchen, then you know exactly how much mess fine powder can cause. You don't want to be cleaning that up from all over the room and the rest of your house, so start by putting drop cloths on the floor. Then, you would have to sweep or vacuum for days to get all the dust up. The simplest way to add some ventilation is to crack your windows. For better ventilation, put box fans in the windows facing outward to blow some of that dust away from you. Keep in mind, though, that screens will become clogged with dust, so you may want to take them off both doors and windows. While you want air movement, you don't want air flow from the drywall room to other rooms, as that will spread the dust around. Turning the air movement off will help keep the dust contained. While shutting off the air will help a great deal, it's also a good idea to block off the return vents. Cover them with plastic and tape around the edges. That way, drywall dust won't be able to sift into the vent, which would spread it around your house and ruin the filter. It's best to move furniture out of the room if at all possible. However, if you can't move it or even if it's just nearby the entrance to the room, throw a drop cloth over it. Covering it will be much easier than trying to clean dust out of it. Put on glasses or goggles to protect your eyes. You will also want to wear a painter’s jumpsuit with a hat. Gritty eyes are no fun for anyone! Also, your lungs will thank you if you wear a respirator or dust mask. Look to see if it's rated for drywall. If it's not, that means the particles can still get through, which defeats the purpose.  If you can't get as much ventilation in the room as you should, you'll need to change out your mask every hour. Just set an alarm on your phone to remind yourself so you don't come out of your work-induced stupor 3 hours later. If you don't want to spend days showering, you might throw some protection over your hair, such as a scarf, hat, or even a shower cap.
Put down drop cloths. Get the air flowing. Turn the air conditioner/heat off. Block off the return air vents. Cover any furniture. Protect yourself.