Question: How do I go about the above (Flying to Daughter's College Town, Flying Back Together: Booking Roundtrip and One-way?)?

I want to fly to my daughter's city, then have her fly back home with me (seated together).

Thank you!

Answer: 
I don't know which country you are in or which airline you intend to use, but many airlines will allow you to select the seat number during the online booking process. Most of the so-called low-cost airlines charge an extra fee for seat selection.
It is possible on most airlines to "link" two separate reservations that have already been made. This effectively attaches a note to the two separately-booked itineraries saying in effect "these two people are traveling together, please seat them together and don't re-route them onto two separate flights if the original flight is cancelled." In general, you will need to call the airline's customer service line to link two reservations in this way.

That said, it is not clear to me (from reading accounts online) how often these "links" are respected, particularly by automated rebooking systems. It can't hurt to link the reservations, at least, but you may still need to be diligent about seat selection and/or rebookings.