What is it?

From a practical sense, it’s writing every morning without fail. That’s it. Everyone has their own requirements when it comes to methods (how much? using what? what about?) but don’t let the details get you down. Just write.
For me, that currently involves a pad of paper and a pen (17 days straight so far). I find it helps me avoid the distractions that a computer might put in front of me. There’s no email or Facebook or Twitter calling to me. If you don’t have that particular issue, feel free to try typing (or use a device that makes it difficult to multitask, like an iPad). Be sure to use a fullscreen editor (such as Byword) to avoid visual distractions and really focus on putting those words down. There’s also something to be said for the tactile nature of writing by hand. It seems to engage a different part of the brain than typing. I don’t know if it’s actually any better for my writing, but I’m going with it for now.

What do you write?

Anything. These aren’t usually pages in the screenwriting sense. I’m not sure I have the brain capacity to deal with that kind of structure 5 minutes after waking (but I’ll try eventually). It’s more of a freewriting exercise (this is the book that first introduced me to the concept). The essense of freewriting is to start writing one idea and see where it takes you. It’s a great way to delve deeper into story problems, character traits, analyze new ideas, etc. I usally start off with some ramblings for a paragraph or so before I really get to the meat of things.

[scrippet] Did that dream have to include a sexy ostrich? Because questioning my bird preference isn’t what I wanted to do this early in the morning. But enough of that. Should I figure out a character from that scifi tv show or break the back of the conjoined twins comedy? Conjoined twins. Definitely
[/scrippet] Total stream of conciousness. The warm start shakes the cobwebs loose and the ink starts to flow a little easier. The challenge is really to bring your mind back to writing when it starts to worry about the day’s obligations (this is where meditation can come in handy). This is why doing it first thing in the morning is useful. It builds momentum for writing/creating before the day tries to bring you back down to reality.

How long?/How Much?

Until your mind runs out of juice or it really is time to get ready for your day. I typically do about 20-40 minutes worth, which adds up to 300-900 words. The popular originator of the concept of morning pages, The Artist’s Way, says to do 3 handwritten pages per day. I say go at your own speed.
The trick is to not stop too soon. Put your pen on paper (or fingers on the keys) and don’t pause for more than 20 seconds. Explore an idea fully. Start writing and go down all the possible paths it opens up, no matter how random or tangentially related it may seem. You’ll surprise yourself with what you come up with.

What’s the real benefit?

So far I’ve solved a problem of character motivation, eliminated a few scenes from a script outline, done a lot of world building for two TV shows, created character backstory, and started exploring ideas for two features and one short film.
Beyond these benefits, it helps me feel productive right out of the gate each day. It also creates a great writing habit. Writing begets more writing, which in turn reminds me how much I enjoy it.

Anything else?

I recommend transcribing your handwritten pages a few days afterwards. That way you get a nice digital copy of things, and you can remind yourself how brilliant you are.
“Why yes, I SHOULD make that character a juggler. What was I thinking?!”