Set Your Criteria
Before you begin training, you need to decide on what your end goal is. Do you want a dog that walks to heel all of the time, or are you happy with your dog walking in front of you on a loose lead?
My border collie is trained to do both – if we are in an area where he needs to be on lead but there is sufficient space, he is allowed to walk in front or behind me as he pleases. If we are on narrow paths, passing dogs / people / traffic, he walks to heel. This distinction admittedly takes a little more effort to train, but it can be done using clear criteria and cues.
Make sure that your criteria still allow your dog to have an enjoyable walk – for example, if you expect them to walk along in heel, making eye contact with you constantly, this might be very convenient for you, but not at all enriching for them. Strike a balance between safety and convenience, and their enjoyment. It is their walk, after all!
Once you have decided on your criteria, you are good to get started.