Even a small dog can be challenging to walk if he keeps pulling on the leash and trying to charge at everyone and everything he sees. Yet, most areas have leash laws meaning you have no choice but to train your small dog to walk on a leash without pulling all the time. The 'heel' command is used to make your dog walk beside you at your heel with slack in the leash or even off-leash where it's permitted, without having to constantly remind your pup where his place is.
Traditionally, the heel position is by your left heel, chosen by hunters. However, you can teach your pup to heel on whichever side you prefer or even behind you if that works best for you. The big thing is to choose one spot to be your 'heel' spot so that your dog always knows where to go when you give the heel command.
When you give your pup the 'heel' command, he should see it as an order to move to the chosen heel position and remain there throughout the walk unless you release him. Most people follow the example of hunters and train their dogs to heel on the left. Hunters did this so that they could keep their gun hand free. Bear in mind the when your pup is in the 'heel' position, he should not be pulling on his leash or trying to wander around, he should remain by your side, matching your walking pace.
This training can be done after your pup has mastered walking on a leash or you can do both at the same time. You may find it easier to train your pup to heel after he has mastered walking on a leash and certainly after he has mastered the four basic commands of 'sit', 'stay', 'come', and 'down'. You can teach any age dog to heel, but of course, the younger your pup is when you start, the easier it will be and the less time it will take.
You won't need much in the way of training supplies to teach your small dog to walk in the heel position. The biggest thing is your dog needs to already know how to follow commands and preferably how to walk on his leash. You will need a few things to make the training go easier:
If you want this training to be successful, you need to stick to it and work with your pup on a daily basis. The nice thing about this "trick" is that you can work on it every time you take your pup for a walk.