The best way really is to just start. You might chop and change, write and rewrite, many times but you will find a way to make it come together. Writing notes and a list of plot points helps, or even writing out the separate sections and then finding a way to make them meet. Don’t get bogged down in the minutiae of sentences and paragraphs. Getting the bare bones down in your starting point. I used a spreadsheet and would add in new points and landmarks as and when they came to me. I still ended up spending ages editing, and adding, and amending until it felt right. Taxing but cathartic to get it all out.