1.) Set sv_cheats to 1 in your console. Make sure the dev console is open, and type "sv_cheats 1". (Without the quotation marks, of course.)
2.) You must have your own server. These don't work unless you are the server admin.
3.) All players must know upon server join that they are playing on a cheat server. This can be done by a script, or just by telling the players as they join.
However, using these cheats in a multiplayer environment, and violating rules 2 or 3, you could be banned, or your server could be blacklisted among the community.
This will NOT result in a VAC ban unless you're doing it on a VAC server without full disclosure.
In my opinion, I'd say not to use these cheats in a multiplayer game at all. It causes other users to not have fun playing the game, and they may avoid your server altogether.
Acheivements will be re-enabled after you restart the program.
Cheers.