Start off working in your enclosed garden, were it is safe for your dog to be off lead.
Walk around with your dog off lead, reward your dog every time they choose to come to you (treats, toys, praise etc) i.e. don’t try to recall them yet, let them choose to come to you.
Repeat this for several sessions, so the dog associates coming to you with something positive.
Now start calling your dogs name, give him a couple of chances to come to you but if he ignores you then go to him, stick a treat under his nose and lure him back to your start position. Touch his collar and then give the treat.
Occasionally put your dog back on the lead for a few steps and let him go and play again.
Repeat this for several sessions, initially working close to the dog and gradually increasing the distance between you and them.
Once your dog is recalling, you can add in a command word “come” or “here” are commonly used terms, but what ever you choose is fine, just keep it consistent.
Once you have 100% recall at home, take your dog somewhere quiet and enclosed outside. Use a long line or flexi-lead for back up control and repeat the above process from the start.
Finally start introducing other distractions, initially dogs that you know and trust and eventually working up to walking in a busy dog friendly park.
Once you are happy that you have good recall, work without the longline flexi-lead
Remember to always carry some tasty snacks (or a suitable toy) as a reward, never punish your dog (even if it takes them ages to eventually recall) and keep the session short and try to finish on a good note when ever possible.
Long term, keep recalling your dog throughout the walk, reward and let them go and play again (this prevents the dog from getting wise to when you put the lead on it means “game over” for the walk.)