For my dogs, what worked was after getting them in the down position I gently rolled them onto their back. Naturally, they would roll over to get back into position. Every time I did that I gave them a treat. Both picked up on what I wanted VERY quickly. Moving my hand to try to guide them into a roll did not work for either of my dogs, so just got to find which technique works best for yours.
Good Luck!
Good Luck!