Steam – appear offline on steam but still be online

Here is an instruction on how to create a server and put bots on it:

1. Make sure you have the port 27015 accessible for connections.
   ^{How? Look here.}
2. Open the game console.
   ^{How? Options⇨Keyboard⇨Advanced...⇨Enable developer console. Press ~ / ` to open the console at any time.}
3. Type sv_lan 0 if you want the game to be public, or sv_lan 1 for LAN only.
   ^{Public server is recommended}
4. Type map <mapname>, replacing <mapname> with the name of the map you want to play.
   ^{Or press the [+] to the right of BROWSE SERVERS in main menu instead.}
5. If the server is public and you don't want anyone to intrude, you can set a password:
   Type sv_password <pass>, replacing <pass> with some simple password.
   _{(e.g. sv_password 1234)}
6. Set the bot difficulty: tf_bot_difficulty <x>, where <x> is from 0 (the easiest) to 3.
   ^{(e.g. tf_bot_difficulty 0)}
7. Add bots: tf_bot_add <team> <class>
   <team>: red | blue
   <class>: scout | soldier | pyro | demoman | heavyweapons | engineer | medic | sniper | spy
   ^{(e.g. tf_bot_add blue heavyweapons)}
   To remove all bots: tf_bot_kick all

Here are some ways for your friend to connect. If one fails, try the next one.

• Join the game via Steam.
  ^{In Steam Friends list press ▼ and Join Game.}
• In console: connect <ip>:27015, where <ip> is your (server's) IP address.
  ^{(e.g. connect 11.22.33.44:27015)}
• Try the same thing with your (server's) local IP address (find it in Windows/Linux)

If nothing works, see the step 1 of server creation instruction.

VALVe recently added the function so that it is possible to earn achievements in offline mode. They did not announce it, though, for some reason. One can only speculate about this.

The condition for this to work is that, while online, you have run the game once or viewed the details in your library. Otherwise Steam wouldn't "know" that there are achievements in the first place.

Source: http://forums.steampowered.com/forums/showthread.php?p=29035241

Also, note that achievements you earn do not register onto your Steam account until you connect back to the Steam service (have an internet connection) and are dated at the time of reconnection.