The way I work is to select the "hero shot" in Adobe Photoshop Lightroom 4, do whatever touchup needed there and bring the complete (head and shoulder and background) shot over to CS6. Next I'll resize the canvas to the anticipated output size. (Usually 16 x 20 or 20 x 24.) I'll make a "canvas Layer" under the hero shot, turn off the visibility of the head shot and start working on the clouds.
The first thing you might want to checkout is that, on an image like this, I label each Layer. Not the Smart Filter or Adjustment Layer labels, they already have labels , but anything that winds up hard to see. Most of the Layers are versions of clouds. Cloud brushes, by definition, are light and airy. Not too much black. Therefore you end up with almost invisible Layers. Either that or, a few posts back, I used Brushes that looked like lightening, or grunge, or splatter or whatever seems to fit the image I'm playing with. If I don't label the Layers I have no idea what's in there. A lightening Brush Layer looks totally blank. The cloud Layers are there, you can tell something is in the thumbnail, but what cloud Brush and sometimes what color was used are indistinguishable from one to the next. So, labeling is a big (huge) deal. Some projects wind up with twenty or thirty Layers. If the only labels are Layer 1, Layer 2, ... Layer 23, ... Layer 35 the only recourse is to flip the visibility icon (the Eyeball) off and on until you find the one you're looking for. At least if I know I'm looking for a Layer that holds Magenta, I harrow it down to the Magenta Cloud Layer, or the Magenta Grunge Layer, or Splatter Layer. I'm down to looking at three or four Layers max. It saves more time that it takes to label them. It's become a no brainer to label them as I go. New Layer, new Brush, new color, new label.
The first cloud Layer for today's image was a green filling. I blew it and didn't make a new Layer. The second cloud Layer was the blue in the fellow's jacket (I typically use colors from the hero image). I wanted it under the green clouds to produce a little more depth. I was clicking on the blue cloud Brush, but nothing was coming up. I realized my error and tried to correct it. The bottom line is that it was easier to trash that Layer and redo the "canvas" Layer and make a new blank Layer for the greens.
Typically I'll leave some "white space" around the edges of the clouds. After I'm reasonably happy with the mottled background I'll turn on the hero shot Layer and work on it. I'll use the Quick Selection Tool (W) and Refine Edge dialog box to pull out the head and shoulders. When using Refine Edge I'll select the On Layers mask to clean up the outline. Too many people fuss unnecessarily trying to get "the" perfect cutout. They tend to drive themselves nuts getting every speck perfect. Using the On Layers option you get to see exactly what the cutout will look like there it will be used. It there's a little "fuzzyness" in an area, but you can't see it, don't fuss over it.
I use Free Transform (CTRL T) to resize the hero shot to fit with what I'm doing, either a straight portrait or a montage. I'll also use Free Transform to resize all the background Layers at once. I probably have the relationships of the Layers the way I want them, so resizing one at a time would be a mess. Click on the top background Layer. Shift Click on the bottom background Layer. Use Free Transform to resize the whole shootin' match at once. (Before anyone writes, you could put them in a Group and do the same. You could make them a Smart Object and do the same. There's probably a dozen other ways you can move them all at once. Pick your favorite.)
Once the portrait shot is selected, start to finish the process probably takes less than twenty five minutes to make one of these portraits. Have fun with it.