To me, the hardest part of writing is getting the first draft done. I'm a good enough self critic to do a reasonable job of editing, but the writer's block is real and awful. The standard advice is to silence the internal critic and write. Of course, it's easier said than done. I'm supposed to be writing a paper. I'm commenting on HN instead.
If the goal of your writing is clear, it's easy to make an outline once the mind map is done. If the goal isn't clear, start asking questions about why it isn't clear and expand the mind map until there's enough there to continue. Sometimes this requires research while constructing the mind map, especially for assigned academic writing. Tag pages with sticky notes or copy data with references into a large research doc as you go.
Once the outline is done, the rest is straightforward. An editor always helps if you can afford one, but you don't always need to agree with the suggested changes.