Thanks Marsha, Iain, Andy and Peter. I'm really pleased that you like it, because it's one of the few I've done that's turned out exactly as I saw it in my head
Mattd85 has asked for a little tutorial; it's relatively basic, and I know there are several ways to achieve the same thing, but this is what I did:
1) Practice the poses, and mark the position of each on the floor (dandelion heads are handy for this!), so that there's no overlap and so that you can work quickly to avoid changing light and not have the clouds move too far!
2) Shoot the 5 shots using a tripod to keep the camera in exactly the same place for all 5. I set the exposure and focussed manually, because it's very very important that nothing changes between shots.
3) Buy your daughter some sweets for doing so well and so she'll help next time!
4) Once the five shots are on your computer, if you make any adjustments to the first one, make the exact same adjustment to all 5. DON'T crop or straighten at this stage because it'll make alignment harder.
5) Open all 5 images in your editing app of choice, in my case Pixelmator.
6) In my case, decide that "P" will be the base image. Switch to the "R" image, and select all (Cmd A on a Mac, Ctrl A if you're silly enough to be using Windows), then copy.
7) Switch back to the "P" image, and paste. Because the two images are the same size (you didn't crop, right?), the "R" will be on top as a new layer, and will be perfectly aligned with the "P" image below.
8) Choose the eraser tool, and a brush with a nice feathered edge. Use this to brush away most of the top ("R") layer to reveal the one below, leaving only your model in her pose. It can be difficult to see where you've been with the brush, so once you're happy that everything's aligned properly, turn the bottom layer off so you can see where the top layer has become transparent.
9) You should now have an image with P and R on it (as long as you've turned the bottom layer back on). If you're completely happy with it so far, merge the two layers to keep the clutter to a minimum in the layers palette.
10) Repeat for the remaining letters.
11) Once you have the finished composite, nows the time to crop and/or straighten, and add any other adjustments you want such as vignettes and borders.
12) Save it or you'll be very very cross!
There is a very good tutorial
here showing another way to do it using the clone tool, with some much better examples than mine