If you haven't yet enabled the console from the instructions of any other of the various guides, I will include it here.

1. Right-click The Stanley Parable in the Library.

2. Click on the Properties option.

3. Under the General tab, click 'Set Launch Options...'

4. Add in -console or -dev console if the former does not work.

5. Click 'OK.'

6. Click 'Close.'

The console is now enabled and is bound to the tilde '~' key by default. You can change the mapping in-game with the other keys.

1. Navigate to C:\Steam\steamapps\common\The Stanley Parable\thestanleyparable\cfg\ or wherever The Stanley Parable is located.

2. In the \cfg\ folder, create a new text file with any name. I'll use the name 'restart.cfg.'

3. Open restart.cfg in any text editor.

4. Add these two commands into the file:
bind "r" "exec restart"
restart

The command within the quotation marks is simply calling the file inside the console, where 'restart' is the name of the file. Any key can be used in place of 'r,' however I choose to use it for the sake of simplicity. I choose to bind it at the start of the file instead of within the console so if you load the game up in the future, you wouldn't need to rebind it. Especially useful if the game crashes midway through and doesn't save.

5. Save the file and close it.

6. In the game, open the console if needed and type this command:
exec restart

This will call the file and load a new game.

7. Press 'r' until you get the desired starting scenario.