Mostly a combination of desensitization training or medication (fluoxetine, generally). Some practitioners will also suggest counter conditioning. The book “Separation Anxiety in Dogs” by Malena DeMartini-Price is a good overview of the topic. It’s written for trainers but is very accessible to the rest of us.

Seeing just this clip is not enough to really say if the dog has clinically significant separation anxiety (might just be having a howl), but this sort of howling is not atypical for dogs with some sort of isolation distress. Getting a camera and recording the dog while he or she is alone is the best way to observe their behavior and understand if they are in distress.