Place the demo file (.dem) into your cstrike directory.
(C:\Program Files (x86)\Steam\steamapps\common\Half-Life\cstrike\)

Run Counter-Strike and enter "viewdemo nameofdemo" without the quotes marks.

If you receive any model errors then go to our website's download directory and download the necessary models.
http://csafm.com/fastdl/models/

Place the downloaded models in their corresponding folder.

Post on our forums and www.CSafm.com if you have any questions.