Cheating can easily be achieved by modifying the game's data while it is running. Modifying game data usually violate a software license agreement that prohibits modifying the program at all. Anti-cheats such as VAC try and stop this, not always working.
The way this is usually achieved is by running another program along side of CS:GO, like Cheat Engine. This modifies values of the game to give you the advantage.

The reason for wanting to send false packets is to achieve the same result as the previously discussed method. Instead of modifying the local runtime, you directly pick certain packets that are being send from your client to the server. To do this, you must create a custom packet sniffer as something like Wireshark will not suffice. Check which packets do what and start sending your own modified packets. You would ideally do this from a spoofed address, which should be on a different LAN than your game client.
After the spoofing a server part is down pat, you can design a piece of software to simulate the server and send false packets. it should be undetectable if you spoof properly. VAC only checks client-side and if the "server" sends packets it wouldn't know. however be careful as if you mess up the spoofing and the real game server and your own "bad" packet server cross, then VAC will most likely issue you a ban.