Red Dead Redemption 2 is coming to PC on November 5, and you can already preload it. What this does is that you can download the game and be ready for launch, but you won't actually be able to play the game until it is officially released. On the release date, you will get a small patch update that makes the game playable, and this is only a few MB, so it only takes a few seconds to download, compared to hours that it might take if you need to download the entire game on launch day.
The first step to preload Red Dead Redemption 2 for PC is to actually prepurchase the game. This can be done from the Rockstar Games Launcher. When you have bought it there, open the Rockstar Games Launcher app on your computer and select Red Dead Redemption 2. You will then see a Preload option where it usually says "Play", so click on this to begin downloading and installing the game.
And that's all you need to do. The game will now download as long as your computer is turned on, and the Rockstar Games Launcher is open (it can be minimized to system tray). Once it is completely downloaded, all you need to do is to wait until November 5 for the small patch that will unlock the game.
Red Dead Redemption 2 requires a hefty 150GB storage space, so if you got a slow internet connection, you might actually need a few days to get the download done.