To me, the hardest part of writing a manuscript is staring at the blank page on my computer. Here are a few tips for how to avoid staring at the white screen of death for embarrassing amounts of time.
Work on your figures. Leave plenty of time because it always takes longer than you think it will. Like way way longer.
Make an outline - then slowly fill in that outline until it becomes more and more detailed.
Write your manuscript like a bedtime story. Start with once upon a time and just write and write and write. Then spend lots of time editing.
Write each point in super short sentences (like six word sentences). Write what you have to say in these super short and choppy sentences, re-order according to logical flow, then later work on your prose.
Write your manuscript in this order: figures, figure legends, methods, results, discussion, introduction, abstract.