Suppose you have a writing assignment to do and it's due tomorrow and you're completely blocked. You don't even know where to begin.
Here's how to get started.
First, accept the general strategy that you're going to produce crap first, then make something out of it later. Because that's how writing works, frankly. Everything you've ever read in print started out as something way crappier than what finally got published. Most of what passes for "writing skill" is actually revision skill.
Secondly, forget about rules. Drop all your inhibitions over grammar, syntax, spelling, vocabulary, use of pronouns (first person, second person, third person), etc., because all that stuff can be fixed later. If you have a brief (80,000-foot-level) outline, fine, but for now take it off the table and hide it somewhere.
Start by writing the following sentence: "The most important thing I'd like to say about [subject] is XYZ." (Fill in the subject and XYZ yourself.)
Again, don't fuss over the fact that you're using first person voice ("I'd like to say"), because that's easily fixed later. There are hundreds of ways to fix it. Here's one: "There are lots of ways to look at [subject]. But probably the single most important thing to note about it is XYZ." Here's another: "Most people think ABC about [subject]. But in fact there are many reasons to take the XYZ point of view. A quick review of the evidence will show why DEF might well be a more worthwhile way to understand [subject]." You can always take yourself out of the discussion. Do it later.
Okay, you've written something, so congratulate yourself. The thing to notice is that once you've captured your main idea in a few words, you can now move in one of two directions. It may well be that your most important point is something you can only get to after first addressing a bunch of other things. In that case, move up to the top of the page (above the sentence you just wrote) and plan on writing downward, until you get to your most important point.
The other way it could go is that once you've stated your most important point, you need to back it up with examples and/or discuss important sub-points. In that case, start writing a new paragraph below the sentence you just wrote and plan on continuing downward toward the bottom of the page.
In one case you're moving toward the main point from above; in the other case you're moving from the main point downward. It may well be that you end up having to do both. But the point is, you've driven a stake into the ground. You have a reference point to work away from, or work toward.
Don't be afraid to state your conclusion first (at the very top of your piece), then, in the next paragraph, back up and explain how you got there. When I feel it's going to take a lot of difficult setup to get to my main point (and then I get all constipated-feeling, because I know the backstory is going to require a ton of well-thought-out explanation), I shortcut the whole process by stating my "punchline" early on, usually in the second paragraph. The first paragraph will state what it is I want to talk about and perhaps give some bullshit justification for why it needs to be talked about. Then, right away, in the second paragraph, I'll say something like "Rather than draw this out, let's cut to the chase. The right way to approach a problem like XYZ is to think about it in terms of ABC." Then I spend the rest of the piece supporting my already-delivered "punchline."
So give yourself permission to introduce topics in any order, including conclusion-first.
It may sound simplistic, but the most important thing you can do when you're blocked is just write something. Laugh, resign yourself to the idea that you're going to produce utter crap, then do just that: Quickly write down a big long list of absurdly simplistic statements about your topic. Or just write a laughably bad first paragraph and pretend you just discovered it under a stack of papers in a mental hospital. Laugh at it. Then move on.