I can recommend this short talk on the writing process by Scott Kubie, which sets out four stages for each 'assignment'.

It's all great, but I want to particularly second the idea of creating an outline before you get started. That could be a few bullet points, draft headings or even a sentence that describes what will go where.

Things might change when you get into the meat of things, but that's okay. Everything changes. Change is good. Life is change.

Scott has a personal website absolutely full of content-related resources that you should also visit and explore.