How do you watch thoughts passing by and not engage with them?

I feel like I'm going crazy here. I've seen this repeated so many times on this subreddit but I have no clue how to do it.

Anytime I notice I'm thinking about something, it's after I've already been thinking about it for at least a few seconds. I get pulled into thought and then realise what I'm doing and return to focusing on my breath.

"Just watch your thoughts passing by like clouds but don't engage with them"

How on Earth do I do that???