Exercise: you know a lot about that

Pick the subject you could talk about all day

Set a timer for at least 15 minutes and write (as quickly as possible and without stopping even for a minute to think) about a subject you know a lot about. Keep writing and adding information until the timer goes off. You can revise it later and make it make more cohesive sense later. For now, just get all of your beautiful knowledge on the page.

But, do note, that there is a difference between having a lot of knowledge about a subject and having great passion for it. Passion, while great, doesn’t bring a lot of substance to the party when it comes to writing or speaking with any degree of authority, so lean towards the subjects you already know a lot about rather than the subjects you want to know a lot about.