To begin with, let your dog off the lead in a small enclosed area that is quiet and empty. Preferably this will be a back garden that they are already familiar with, but it could also be a friend or relative’s garden, or even a small enclosed space in a park that’s empty and relatively quiet. While they are off the lead in this enclosed space, practice calling your dog’s name and getting them to come back to you. Try using treats to coax them back to you and even better, practice getting them to sit so once they are with you they are attentive and still.
Take them on plenty of walks with loud noises and other dogs, to get them used to other people, other dogs and loud noises such as traffic. Try letting them off in an enclosed area with other dogs, there are many instances of dog training facilities having secure areas for dogs to play in, or you could even let your dog off with friends’ dogs in your own back garden. Once your dog is reacting to your voice, and is fine with basic commands and recall, it’s time to let them off the lead, if you’re still apprehensive, try letting them off on a walk which is particularly enclosed at first. It could be a fenced off footpath or even an enclosed wooded area where they can’t run into any danger.