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: 
Within the US, there isn't round-trip pricing any more on most airlines; the price of a round trip is just the sum of the two one-ways. So you should buy:

* booking #1: a one-way ticket for you from your city to where your daughter is;
* booking #2: two one-way tickets for both of you for the return trip.  

On the return trip both of you will be on the same reservation, so you should be able to sit together.

If this isn't a US domestic flight, I'm not sure if there's round-trip pricing, but you can easily get prices for tickets and see if there is or not.

The advantage of doing it this way (as opposed to buying a round-trip for you and a one-way for your daughter) is that if something goes wrong on the second leg the airline will make efforts to keep the two of you on the same plane.

(As it turns out I just got back from a trip within the US exactly like this, and I booked it in the way I described.)
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.