Cocker Spaniels are an incredibly cheerful, easygoing, and playful, which is part of what makes them such popular pets. Along with this, Cockers are very intelligent and easy to train, especially when you start training them while they are still puppies. As with most breeds, successful training requires plenty of time, repetition, patience, and continued positive reinforcement.
'Heel' is one of the most important commands you can teach your pup beyond the four basic commands of 'come', 'sit', 'stay', and 'down'. The traditional 'heel' position is on your left side while you are walking down the sidewalk. You can teach your pup to heel on the right side if it works better for you. This position is by your side, not in front nor behind you. Your pup should be close enough that he is virtually brushing up against your pants leg and there should be no tension on his leash.
The 'heel' command tells your dog that when he hears it, he should immediately assume a position by your left leg ready to go for a walk with you. It also means this is the place where he should be anytime you go for a walk unless you give him permission to do otherwise. It does NOT mean he can walk behind you, ahead of you, or be trying to pull you along. Heel puts your dog in position to remain in constant communication with you so that he knows what you want the minute you want it.
You can teach 'heel' to virtually any age dog, but the sooner you start the better. The ultimate goal is for you and your pup to be able to go walking together whether he is on or off his leash with him maintaining the heel position, no matter what.
Training your Cocker Spaniel to "heel" doesn't have much of a supply list. This type of training is more about being patient with your pup and being ready to put in the time needed to achieve your goal. Your job is to work with your pup until he understands what you expect of him when you give him the 'heel' command. However, you should plan to have a leash and a good supply of treats on hand.
Also, training always goes better when you have a quiet time of day to work with your dog. Fewer distractions make it easier for your pup to focus. Beyond that, the only other things you need are plenty of patience and lots of time to take your pup out for lots of walks on a daily basis. Remember not to punish or chastise your pup if he doesn't get it right, just keep working with him until he does.