Overview
This guide describes the simplest way to get started with importing any Source 1 maps. (Half-Life 2, CSGO, Portal & Portal 2, Left 4 Dead 1 & 2, any other source 1 game) works for all.
Decompile the map
Every source 1 map has a .bsp file. In your Steamapps folder (usually c:/Program Files(x86)/steam/steamapps/common/-) look for the folder containing all the maps.
Once you’ve got your map, you will need one program: BSPSOURCE.
BSPSRC can be downloaded here: [link]
It is in the form of a .jar file (java file) so make sure you have installed Java Runtime (https://java.com/en/download/)
Once you have it downloaded, open up bspsrc.jar
You can add as many as you like, I’ve only added one.
Click Decompile and find a location for your new files.
They should be .vmf files. These can be imported into Half-Life: Alyx.
Open up the Hammer editor. use File>Open or Ctrl+O as if opening any Half-Life: Alyx map project.
Change the filetype (circled) to Source 1.0 Map Files (.vmf) Now you can select and open your map
Issues
After doing this, you will still need to import textures and models, as well as fix the texture scaling. This guide currently does not cover these, you will need to find more information on these subjects