I asked my tai chi teacher about Taoist meditation once and he told me it was very advanced stuff - not for the dabbler. I pressed him on it, expecting to hear about a complicated procedure with tongues curved in a particular way, perinium clenched, back straight, all sorts of stuff I'd come across in books. But he only said "just think about your whole body and nothing else."
Try that. It's harder than it sounds. You'll probably find yourself thinking about only your toes or your ribcage, having to remind yourself to expand your awareness. Maybe the laundry will pop into your head. But if you keep at it, it's pretty effective.
It doesn't really matter if you sit up. If discomfort is a problem, do it lying on your back, or walking, or if you're more comfortable in a chair than on the floor, use a chair. If you feel like moving, move.
Edit: I should mention that I never do it for a fixed number of minutes. I do it when I feel like doing it, and continue until I feel "done". Sometimes that's 10 minutes, sometimes it's much longer - especially if I'm in a beautiful spot outdoors with nothing else on that day and nobody else around.