I had to do this so I could skip the intro ride, as it unfortunately crashed every time Wheatley would attempt to 'dock'.

Here's the process:

- find Half Life: Alyx in your library
- go to Properties
- General
- look where it says Launch Options
- type in -dev in the bar below
- launch game
- when loaded in the modded level, press the menu button three times to access the modding menu.