I sort of alternate between a few different methods.

From the get go I knew what type of story I wanted to write. I've always been a fan of science fiction and dystopian novels in particular (1984, V for Vendetta, Fahrenheit 451, Brave New World to name a few). So in that sense I had already sort of established the basics, but how could I make it uniquely my own? I made one of my protagonists a gay man in his 20s (like me), cause if I were a reader I would love to read a dystopian series with a MC who I could relate to/see myself in. Additionally, I knew I wanted the setting for this dystopian society to be set in the near future (roughly a century) and within America.

I've been really inspired by a number of events that've been going on around the world in the political scene, which gave me ideas for events that occur in the plot of my story. Rather than start off within an authoritarian regime, I much preferred the idea of starting off with this futuristic America going down the path towards authoritarianism over the course of the story (like Germany right when the Nazi party were coming into power). Which gives me lots of opportunity to weave in antagonists like corporate interests, white supremacists, religious extremists etc. And characters like my gay protagonist would be roped in with elements like an Anonymous-like hacker group and others who are fighting against corruption and the rising fascist movement.

It definitely depends on the genre/type of novel you want to write, but at least for the type of story I'm writing real world events and history have been a big help in giving me ideas on conflict and other obstacles the main characters need to overcome.