Where do you....

Place the air stones in a DWC. Do you place it at the bottom or on the sides in if so how do you keep them placed?

weigh it down to the bottom under the center of the root pot

if you have a large res you can secure some to the side if you want. Ideally you want it down at the bottom in the middle. I use several so that each part of the root system gets a equal amount of oxygen

If it is a round airstone put it on the bottom but if it is straight and too long stick it to the side aimed to the middle of the bucket