How to watch a match demo

1. Download the demo:

Go to the completed custom matches page: Click to open

On the page, select the match whose demo you want to download and click “Download demo”.
The download of an archive containing the match demo file will start.

2. Extract the archive and move the file
Open the downloaded archive.
Extract the file with the .dem extension and move it to your game folder:
Steam\steamapps\common\Counter-Strike Global Offensive\game\csgo

3. Launch the demo in the game
Open the game and enable the console (if it’s not active, enable it in the game settings).
In the console, enter the command to start the demo:

playdemo file_name