Well this is how I trained it.
I taught my dog down first, ala sphinx
I then taught flat ie laying on side with head on floor
I then took a treat and put in front of the nose and then lured by moving the treat down to elbow and then moved it over towards floor
Once the dog did this several times on either side I added the cue from the flat position.
If you want to train the roll over from the stand or sit then you need to chain each behaviour into a sequence.
HTH