Heeling is one of the harder obedience commands to teach a dog. Heel is not just having a dog walk next to a trainer, it is getting the dog to realize that heel means they need to stay in a certain area of space around the trainer. That space is able to be put anywhere an individual likes but it is commonly on the left side of a person.
The easiest way to teach heel on either the right or left side would be to use a leash and collar to loosely walk your dog. This is called loose leash walking where the dog can be anywhere that they would like. After walking for a moment and setting a nice pace approach a wall in which you position the dog to be between the wall and yourself. This forces the dog to assume a position beside you and will not allow them to move vertically away. As soon as the dog is in this general area give them the command heel and immediately reward because they are already in the position. Using the wall for a few repetitions is very useful in helping the dog understand.
After the dog is doing great on the wall you can start to move away from the wall and simply giving the command. When walking you can use your hand as a lure to bring the dog back to your side by holding a treat, patting your leg, or petting them when in position.
As you and the dog become more comfortable doing this exercise you can have the dog sit and then call them into a heel instead of using the loose leash walking technique. To do this simply have the dog sit and adjust slightly so that the dog is in position, you take a step and call the dog into a heel. Once you get a step or two steps and they’re still in heel free the dog from the position and reward them generously. You can work on duration from this point and only reward them when they stay to your side for the amount of time you are requesting. But also be aware that if the dog can only take two steps before it wants to break from heel, you may need to go back and do some more easy rotations