Figuring out where your feet will be is the easy part. Simply note a detail on the ground and go to that spot. Alternatively, place something on the ground to mark the spot you wish to stand; make sure it's not too conspicuous (for example: a twig or leaf or rock that doesn't look out of place).
Figuring out where you head is going to end up is a little tougher. Here's how I set up a scene:
- I set my tripod to half my height (I suppose if you're short and your tripod is really tall, you could extend the tripod to your full height instead). I place the tripod in the spot I think I want to stand.
- With my camera, I walk to where I think the tripod will eventually be set. I look through the camera at the scene. To imagine where my head might end up, I "double" the height of the tripod. Generally, I'm looking for a "clear" area (lake, grass, whatever) so that my head doesn't disappear into a mess of detail.
- I'll fiddle with the tripod (which is still representing me) and my camera position until I'm satisfied with the result. I'll mark both final positions with a small twig.
- Next, I set up the tripod at the camera position, adjust, and lock down. I set my self timer to an acceptable length (typically 15 seconds). Even if I'm not using fill flash (rare), I'll mount my Speedlight on the camera (and point it off-scene if it's not being used for fill), and set it to fire on Rear Sync.
I trigger the self timer and move to the position I marked. If I'm facing the camera, I hold my position when the red ready light stops blinking and goes solid, and hold my position until the flash goes off. If I'm facing away from the camera, I hold my position from the time I arrive at my marked spot until I see the flash fire out of the corner of my eye. Generally, I've found it easier to see the flash than to hear the camera shutter.
From the above, you may guess that I do a lot of shooting before and after sunrise. You're right. And I've found that I can typically hold a pose for 10 seconds without a problem. However, note that if you're going to hold a pose, you may need to exaggerate it a bit, especially if you're pretending to be walking. Don't be afraid to shoot a half dozen variants, some with exaggerated poses.
Of course, these days, we have some cameras with flip-LCDs so that you can just set the camera to Live View, flip the LCD, and look. That works if you're close enough to see what the 3" LCD is showing you accurately, but that generally doesn't apply to my shooting.