If any of you have done this or know how to recommend doing it I'm listening.
Been doing it for years. Always fun not to need track power.
RCS in Australia have some "entry level packages" on this page:
with a 1-page instruction sheet here:
It is basically a simple handheld transmitter, and a receiver and ESC (electronic speed control.) Connect the battery, plug in the ESC to the receiver, and connect the ESC output to your motor. Done.
I would suggest picking a loco that has a bit of space inside it for your first attempt. Most tender locos have enough room for all the bits, so all you need is a couple of wires (and a plug) between the loco and the tender.
Even better: the exchange rate is now almost 1.5 AU$ to the US$ so the price is right!