I write what I'm currently feeling and what is on my mind at the moment. Writer's block is a sign of trying to force something, which never works. Start with what's going on at the moment, and there's never any writer's block.
When I first began writing, there were no computers or word processors. There were only typewriters, which meant you had to start with Page 1, and then continue writing in succession.
I'm so grateful that, with computers, you can write a book in the middle, or the end, and then write the beginning. This is how I now write: whatever topic I'm fascinated with at that moment. So, I may write Chapter 5 today, and then work on Chapter 14 later, and then work on Chapter 1 tomorrow.