I have used two different techniques with the dogs over the years. It depended on how stubborn the dog was on which one I would use. The first, I used a cookie, of course, and told her to sit, then as I repeated "down" as I brought the cookie down in front of her and pulled it back. If she tried to stand or walk, I pushed her butt down and tried again.
The other way, I put a leash on her, and as I did the previous cookie thing, I pulled her down gently with the leash, when she reached the floor, I gave her lots of love and the cookie, and repeated it again.
Both ways were taught to me by trainers, and worked.