The best thing to do is to manually select the focus point closest to what you want to be the point of focus, and if necessary recompose only slightly from there. That's because turning the camera to recompose moves the plane of focus more than you might think — see this answer for a nice diagram.

Typically, with portraits, focusing on the eyes is considered "correct" — most people looking at portraits respond well to that, generally with the closest eye in sharpest focus. (Long ago, was a fashion for focusing on the cheekbones rather than the eyes, but these days general consensus is on eyes. Of course, you don't have to follow that.)

So, put the camera in focus point selection mode. Pick a focus point close to the subject's eye, put it right over the eye, lock focus, recompose slightly if needed, and click.

Your Pentax camera makes this easy, although it is not the default — the four-way arrow controller defaults to bringing up various functions like drive mode, white balance, etc., but if you press and hold OK, there will be a beep, and the camera will switch to moving the focus point, which can be very quick once you've practiced a bit. (Press and hold again to put it back to the function mode.)

Some cameras — usually mirrorless cameras which work with the full sensor for contrast-detect autofocus, or DSLRs in liveview — can do eye-detection autofocus. If this is an option and your camera can do it quickly enough, I highly recommend it. That lets you concentrate on the subject's expression and timing the shot rather than on making sure the focus point is aligned.