Waterloo Train Station to Windsor Castle.. Transportation?

Take the train. It's a 10 minute walk from the Windsor and Eton train station.

South Western rail from Waterloo - direct trains every 30 mins.

When I was there in 2015, Waterloo to Windsor was the worst way to go, the train went way out of its way, a commuter rail for suburbs forever, and came in to a 2nd Windsor station that was not near anything at all. Going Paddington to Windsor was more direct and comes in to the station that's right in the middle of the town.
There were also green busses that went from Victoria to Windsor that dropped me off right in the middle of town near the castle. Perhaps something has changed, but be sure of your stations and how to get to your hotel.
Have fun!
-Alison

If your starting point is Waterloo station then taking the train from there to Windsor & Eton Riverside is the obvious way to go. The trains run every 30 minutes and take 53 minutes. And you can now use a contactless card to pay for the trip, but not an Oyster card.

The trains from Paddington are quicker, taking between 28 and 38 minutes, but you need to change trains at Slough, although it's very easy. But the shorter trip will be negated by needing to get from Waterloo to Paddington to catch it.

Both stations are very near central Windsor and the castle.

Train from Waterloo to Windsor & Eton Riverside is how I always go. And the station is hardly in the "middle of nowhere." Walk out the station, look up, and the castle is right there. Windsor & Eton Central is only 3 minutes closer at the Windsor end, but from where you are starting, a half-hour longer at the London end.