How to Record and View Demos

To RECORD a demo:
  Step 1: While in game and Spectating the person you want to record, open your console and type in "record user_name". This will start the recording.
     
  Step 2: When you have enough recorded, in your console, type "stop".
     
  Step 3: Once the demo is stopped, it will be saved in your steam folder. You don't need this but you may want to know where it is.
      - Usually it is located at "C:\Program Files\Steam\steamapps\user_name\counter-strike source\cstrike"

To VIEW a demo:
  Step 1: Disconnect from the game. Then, in your console, type "demoui" and hit enter. You will get the following window...
       
  Step 2: Click the "Load..." button and select the appropriate demo file.
      - You may have to browse to your steam folder as stated in "To Record a Demo > Step 3.
     
  Step 3: Once the demo start, you can watch it.
      - To make it a bit easier to see if they are walling, you can use "sv_cheats 1" and "r_drawothermodels 2".
      - NOTE: sv_cheats is ONLY for use when viewing a demo. You can NOT use it in game so don't try or the server "might" kick you.

Here's a nice video if you are more of a visual person

How to record/play a demo in Counter Strike Source