Since I have two dogs, I may either alternate dogs or do several sessions in a row with one, then several sessions in a row with the other. I'm planning to make my sessions either 10 repetitions or three minutes, whichever comes first. I'm also keeping track of repetitions, marking the errors, so I'll be able to tell easily when the dog is ready to have the criteria increased. I'll also be able to see when I've increased the criteria too fast because my success rate will be low and will remain low for two or three sessions. Training is a mechanical skill.
Timing is critical.
Basically, have someone drop a ball or a set of keys. You try to click so that your click and the sound of the object hitting the floor coincide. You shouldn't be able to tell them apart. Have the person drop the object from varying heights. The game gets much more difficult as the object starts closer to the floor.