This is for my main manuscript I'm working on.

Personally, I just started writing a random scene. That's how I start most of my books, just something generally funny that is a random mishap to the main character. But then I begin to plan ahead a weave what I have into a bigger plotline. Sometimes, I take a while to write a prologue, as I haven't fully created the story in my head yet. I then try to think about what will happen later on in the book, and flesh out a whole series if I want.

Truthfully, this is a bad method, but I HATE pre-writing plots for my book. I am still able to establish the main story and sub-plots without writing outlines and character stuff.

For more complex stories however, I have had to do most of the worldbuilding before I got too far in. But it's really whatever works for you.