Step1: Get their Steam ID by type "Status" in your CS game console (Press ESC from your keyboard).
Step2: Record the demo with this command ----> "
record FILENAME.dem" in your CS game console.
Example: record cheater.dem
(If you do it correctly, after your type that command, you should be able to see this message from the game console: "recording to cheater.dem".
Step3: After that, click "Resume Game" or press "ESC" again to record a demo (make sure u joined as inspector, so it will be more easier for u when u record it)
Step4: When you finish recording, type "stop" in your CS game console.
Step5: To view a demo, type "playdemo FILENAME.dem" OR "viewdemo FILENAME.dem"
Example: playdemo cheater.dem OR viewdemo cheater.dem
Step6: Demo file's default location:
C:\Program Files\Steam\steamapps\YOUR STEAM ACCOUNT NAME\counter-strike\cstrike
Step7: Attach the demo files or upload the demo and post it to DNS forum ^-^
And here is the link of the tutorial for "How to upload the demo"
https://dnsgtforum.forumotion.com/t110-how-to-upload-demo-s