It gets asked all the time, I'm probably not alone here. The answer is not simple. It's far from simple. I just do, next question...keep with the "mystical" artist image. LOL. I will try and answer the best I can, but I can almost guarantee it won't slake your needing to know the answer.
I do get ideas in my noggin'...stuck in there, mucking things up. 95% of them, don't even make it onto the canvas. Why, you ask? Seeing something in your head and actually getting it on paper (for me, at least) is almost an impossible feat. I think there is a short circuit between my brain and my hand...but it goes deeper than that. What you're seeing in your mind is 3D, vivid, perfect...what you can do in a 2D medium (even digitally where I have the access to 3D) is a horse of a different colour, we'll say it's an argyle horse. That 5% (and I'm being generous with the percentages) are akin to a miracle occurring, and even then it's not 100% of what I was seeing. It's frustrating, annoying, can cause you to get that pesky and feared artist block if you try too hard.
I try (keyword TRY) to just let all the ideas go...and just see what happens. (this is the part where your need for the answer will not be satisfied) I may start a session with a very loose idea, but generally speaking I just stare at the "canvas" (in quotes because it's a digital one) and just make the lines, hit undo, start more lines, hit undo, etc. Until something in my noggin' says, "OH! I like this!". Then I continue to go with it. Sometimes it's easy, other times it's not. It could be hours before I get a good start.
I do warm up with just scribbling on paper, circles and swirls...it helps me loosen my death grip and causing me to damage my Tablet. I admit, I get very tense, the lines aren't loose at first. My Wacom has grooves in it from the start of my digital adventures from the nib (just the standard one at that point) digging into the surface...poor poor Wacom. I now use a felt tip, springy nib...which I love. I have a few others to try out, but so far the winner is the one I use now. So warming up is very key for me...the circles and spirals loosen my grip and my mind of all the ideas. Gets me focused. That and usually music or a internet radio show I listen to.
I know it's probably a craptastic answer, I get asked a lot. I can't explain the process, it's almost automatic, when it's not automatic the stuff created is usually crappy and ends up in the recycle bin. For me I can't force it, I have to let it flow, even then it can be crappy.
Yes, a blank canvas is intimidating at times...but other times it's where is this canvas going to take me? I think every artist is different in their process. My process is a mixture of logic, clearing out the muck in my noggin', then just seeing what happens.
This is my process for my abstract/fractal whatever you want to call it. Commercial is a bit different as I'm given the idea from the client. I get to try and create what they are seeing. It's a very fun challenge, and I usually get a ton of ideas. Let them choose, then work on the finals. I add my own ideas in, if they like them awesome, if not, that's okay, while I'm creating it and putting my love of art into it, it isn't mine, whereas the abstract is all mine...I do share though.
Well I hope that was a good enough answer, because it's all I've got. Seriously. I have tried to concentrate on how I'm creating, when I'm creating...It just happens, simply put. I joke and say, "because I'm a sorcerer"...but that doesn't fly usually. So, I hope my attempt at answering the elusive question is good enough. If it isn't, feel free to use the comment section, I'll try and answer the best I can. ~Amanda