I’ve gotten pretty decent at it three ways:
1) watching others stream, such as Markiplier. He is good at channeling his persona into an entertaining commentary while still being focused.
2) Practice, practice, practice. I find it’s helpful to review some of my streams after I do them, like spot check and think about what I liked and didn’t liked.
3) ask friends to watch some and give feedback on what they enjoyed and didn’t enjoy. When did they feel engaged and when did they find themselves easily distracted?
Freebie: I find my best commentary is when I can manage to keep the viewers focused on an objective while adding in comedy and personal reflections to fill the space between actually doing stuff in the game. Not every moment will be action packed explosions, so I try to go with the flow of what people are talking about in the comments and also stay aware of potential things in the game that could be comedic gold.