So, you want to make a demo, eh.
First step: Find your subject and watch him/her in first person.
Second step: Go into console and type 'record hacker'.( note: Hacker can be replaced with whatever you like. It will be the name of your demo)
Third step: Go into console and type 'status'( note: this will display all of the users, their STEAM_IDs, their frags, time, and so on)
Fourth step: Find the players name that you suspect of hacking. Highlight the name and STEAM_ID. Right click, then press 'copy'.
Fifth step: In console, once again, type amx_chat(this is the admin only chat) then paste the name and STEAM_ID of the suspected player. ( note: this is a good way of keeping track of the hackers and demos)
Sixth step: In console type 'STOP' when you are done your recording.( note: the longer the demo, the larger the file. So, for para's convenience, the shorter, the better.
Now that you are done recording your demo, you can watch it by typing 'playdemo hacker' in console.
You will find the hacker.dem (or whatever you named your file) file in your cstrike directory.
Example: C:\Program Files\Steam\SteamApps\email@domain.com\counter-strike\cstrike\hacker.dem