To initiate the pre-load through Steam, please visit your library, select Grand Theft Auto V and left-click the 'PRE-LOAD' button. After setting up the download options (install location, shortcuts etc.), Steam should start the pre-loading process which can be paused and continued at any time you wish without losing any of your download progress. If the 'PRE-LOAD' button is not showing up in your library, please restart Steam and try again afterwards.

PLEASE NOTE:
Pre-loading won't allow you to launch the game prior to its official release date! It will only help you to download the game (or parts of it) beforehand so that you can jump into the game after its release as soon as possible.