How does River Song do her time traveling?

Question

In The Husbands of River Song (the 2015 Christmas special), River Song says she has often stolen the Tardis, did her thing, and then returned the Tardis to where/when it was, without the Doctor ever knowing anything about it.

Is that really her only method of time traveling? Stealing someone else's time machine? Then how does she get to that town, at that time, in the first place? And if she has a means of time traveling of her own, why does she need to steal the Tardis?

I'm doubtless missing something obvious. What is it?

Answer 1

River has shown a number of ways to time travel without the TARDIS, but I think she was talking about something specific in that scene.

During the events of "The Pandorica Opens", we see River buying a vortex manipulator from Dorian Maldovar. This is the time-travel device that Time Agents use to time travel. This allows her to travel to several time periods in that episode to finally meet the Doctor. It's unclear what happens to this device after the events of "The Big Bang", but presumably River could just buy another one if she wanted.

However, it's also important to note that The Doctor takes the TARDIS to River on a regular basis, both while she was in prison and continuing later on. These are how the two of them spend time together in their "marriage".

My impression was that River would often wait until The Doctor was asleep/distracted/preoccupied during one of these trips, steal the TARDIS, and return it to him before he noticed it was gone.