This is how I begin the commands ~ over time I build on it. Some of it is a bit abbreviated because I'm short on time.
Sit ~ I lure with a treat. I show the dog the treat and then raise it up and forward so that he comes forward into his sit. With a squirmy restless puppy, I'll also put my hand on their rump and then slide it down the back of their legs and gently fold them into a sit.
Come ~ I teach this while I'm playing with them. I'm usually on the ground, being very inviting, maybe with a treat or toy, maybe not. I wait until the puppy looks in my direction, and I say "come" and I encourage them to get to me. If they are distracted, I get up and run away ffrom them.
Stay ~ I start with either a sit or a down (depending on which seems easier for the dog) at heel, and I pivot to right in front of them, and then pivot back to heel. The next time, I pivot in front and wait a moment or two before I pivot back to heel. Over time, I gradually increase the time or the distance and then both, as well as once they understand to stay, I begin walking around them to go back to heel.
Down ~ I usually start with the dog standing and I use a treat to lure him down, into a sphinx down. If I'm working with a squirmy puppy, I'll generally keep a hand on his back, but I don't really use it to push him down, but more to block him from going in other directions, like backwards or sideways.
Heel ~ I start with teaching the dog to follow me, by running away from him any time he heads in another direction. Then I teach loose leash walking, with a command of "With me", which is the same thing, but the distance the dog might be able to get is predefined by the length of the leash. I don't teach actual heeling until I have good attention while he is sitting as well as some attention while he is on a "with me". When I begin heeling, I start with him sitting at heel (if need be, I put myself into heel position). I carry a treat or a toy in my left hand, say heel, and step off. If he steps off with me, he gets the reward. If not, I back up and try again. I start with one step, then two steps then start gradually increasing the steps. I want to be sure to maintain focused attention on the "heel" command.
Drop it ~ I teach it as a game of trade. I have something better and when he releases he gets rewarded.
Leave it ~ I hold a treat in my right hand and show it to the dog. I also have a second treat in my left hand. When I show him the treat in my right hand, as he goes to try to take it, I say "leave it" and I close my hand around the treat and take my hand away. Usually it only takes a few reps and the dog begins to be reluctant to look at the treat I am showing him ~ I'm not falling for that again. At that point I reward with the treat that's in my left hand.