For down, I would start in a sit position. One trainer had me in front with the treat being offered progressively closer to the ground, NEVER repeat your commands. That didn't work for Harley. She did better starting in a sit, with me on her left. I'd kneel, give the command (this is after she knew what I wanted) and reach across her back with my left hand, and slide her forepaws out gently, when she's down say Yes, down. good girl whatever. I also usee hand signals with every command. I point to the ground for down. Personally, I would master down before roll over, it's a much more useful command IMO. I use down to reinforce my alfa position (the long down), when greeting small children (very non-threatening, can't jump on anyone, not that she would), and during dinner. It's also effective while waiting at the vet & I could go on & on really. Wait is the other command I use ALL THE TIME.