If I were in that sitch, I would just say "Keep your body relaxed but your spine straight. Pay very close attention to each in and out breath. Try not to get distracted, and if you do, try to realize when it happens quickly and bring your mind back to paying attention to your breaths"
Insert whatever is specific to your particular practice, like eyes closed or half-open, particular hand position, whatever.
I definitely think that any beginner should begin by trying to develop samdhi before prajña, so... anapanasati is probably the best. It's also probably the simplest in terms of explanation.
