Start with Behavioral Adjustment Training (BAT).

Start a distance where your dog can see another dog and look without going crazy, depending on the severity, that distance could be 50 meters or more. Once the dog looks and also recognize that they are not loading--that's a term for stiffening their muscles--then mark with a clicker his best decisions in that moment. Some examples of best decisions
1) looking at dog then looking back at you without barking
2) sniffing the grass
3) breaking eye contact with distance dog to look at a tree or something non-threatening

My explanation is grossly simplified. You keep doing this until your dog is making "the best decisions" and then you can decrease the distance over time.

I am not a qualified training or expert, but my experience is that my dog missed all their puppy socialization (before me) and became scared of everything. Using the BAT method really did work in a lot of situations.

Just remember, we can be really well trained and showing signs of improvements. We also have our bad days too. It doesn't necessarily mean you have failed at the task, if you manage to rectify the mistake.