...have her drop on a dime if necessary.
I am unclear if the other posters read the same thing I did....the OP is asking about a platz or down command, and most everyone is talking about recalls and other behaviors???
Anthony, I believe you're thinking is aligned with how I train....certainly a strong recall is key, but developing your dog to a point where he/she will drop on command it a very important safety behavior to me. I use the German command "platz" but certainly down, lay or any other word employed consistently will work.
To begin, with the dog in a standing position I use food to teach the behavior, and either a clicker or the word "yes" to mark when the dog has it correct. For me, the mechanics of how the dog downs and the specific position is very important, but for a family companion, simply achieving the down should be sufficient.
With a piece of food in your hand such that the dog knows it is there but can't get to it...lure the dog to the floor such that he/she lays down to sniff and lick the hand that holds the food. As soon as the pup drops to the floor, mark the correct behavior with a click or "yes" and immediately reward with the piece of food...on the floor in the down position....then free the dog. Repeat this process over and over for brief intervals (10 mins at a time or so) over what could be several sessions over several days....or could be one or two sessions in one day....depends on you as a trainer and the dog.
Once you're at a point where the dog is anticipating the behavior such that he/she drops to a down position at the moment you start to move your hand/food to the floor, you're ready to put a name to the behavior.
At this point clarity of command is important. Say the word/command, offer assistance, mark the behavior, and provide the reward. "Down", motion your hand/food to the floor, when the dog downs say "yes" or click, and immediately offer the treat. If you perform this in any other sequence, the dog's learning will be hampered.
Once the dog begins to anticipate the behavior on the verbal que, this is when you start to fad the assistance with respect to the physical motion toward the floor with hand/food. Key is to keep up with the marking and reward.
Once the dog has learned the behavior, stay aware of speed. You want the pup to drop quickly, so once you're confident they have learned, and know the behavior, you hold back reward for a slow, leisurely down....say "no", withhold the reward, and on the next que, offer the assistance at a rapid pace such that the dog works to keep up with the hand/food, and when they quickly drop with the hand/food, mark and reward with excitement....but no so much as the break the down.
Once the dog has learned the behavior, you'll want to put a command on allowing the dog to get up...I use "free". They quickly learn that "free" means they can get up and move about.
Another aspect to work on is duration and proofing. Once the dog knows down, start making them hold the position for longer and longer periods of time before freeing them. At first the down should be just a moment, then free....later, minutes, then free, and eventually 15 minutes - half an hour....or until you tell them otherwise. Proofing is when you're at a point where once the dog is down, a deer could walk by and they should not move.
Best of luck.
I can't believe I've recycled this photo yet again...LOL.