Down command can teach by the following method :
we persume your pup already pick up the Sit command on first lession. Put your pup in sit position, hold a treat in front of his nose and eventually move the treat downwards. As the pup nose follow the treat, move the treat in front of its body until the treat reach on the floor and continue moving the treat forwards until the pup lie himself down on the floor to keep his nose contact with the treat, give firm command : DOWN and rewards + praise immediately.
Repeat the commands until he can responds only to word alone. Repeat the command and extent the reward timing :reward / praise them with only after few second they remain in the down position.
Since your pup already learnt the SIT you can mix with the command Down during exercise to let him learn more focus of the command you give before he can move on to the next trick. Please don't keep on Sit.. down.. sit.. down.. many times with same sequence then only reward them, this will cause the pup very bore and don't know why the master asking the same thing over and over again for no purpose.
Problem solving :
1. for pup trying to jump on the treat instead sit properly before exercise, you can hold his collar with your left hand and hold a treat with your right hand and put the treat in front of his nose then only start the exercise above. This will prevent him for jumping up.
2. Some puppies just refuse to down, they will just stand or bow instead down to reach the treat, for this problem, you can sit beside him and lift his front 2 forelegs start with begging position and follow by lower down the puppy into lying down position. The moment he is in the correct position give firm command down follow by immediate rewards and praise.
3. Puppies refuse to stay down, you can put on some pressure on his shoulder muscle for few second then only release and say 'OK' follow by rewards + praise.
** There are no fix training methods, please be creative and use what is work best for your puppy. What we provide only base on general knowledge we know and just for your reference only. Different puppy have its own character and you must always know the best way your puppy will respond well on your method and don't feel bore on the training session. Happy training.