The down can be taught in much the same way as the sit, with food treats being used to encourage the dog into a comfortable position.
Useful if you wish to leave a dog in position for a while, the down is comfortable for a moderately extended period.
Starting from the sit, the dog can be encouraged with a (very small) treat to stretch forward, and following its nose will extend into the down.
The verbal command is usually the word “Down”, said quietly and normally at close quarters.
The visual command is to point to the ground, again normally at close quarters.
The whistle command can be anything you like as long as it does not conflict with any of your other handling whistle commands – I tend to use a long quiet note.
One reason for doing this command at close quarters is to make sure the dog actually is comfortable – it is not fair to ask the animal to lie on an anthill or in nettles or thistles.