Pretty straightforward just in case someone was wondering. Once you start your solo server run the next commands on the console:

sb_enabled 1; (this allows bots to be spawned)
sb_add; (add as many as needed)

Do not use these old commands, will likely crash your game as they use the old l4d2 bots, the new ones are based on Counter strike GO.

This is the new command you should use to add a bot once the round has already started (this is to allow you to spawn bots):

bot_ai2

Then to add 1 bot each time use the command:

bot_add


Credits to @Hellektronic. You can set the bots difficulty using the next command:
bot_difficulty 0-3
0 being easy and 3 expert.


You are also able to kick bots (just keep in mind, each kick takes one bot out, if you kick a bot and add one back, they will either come back as zombies or a cockroach depending on your host rules, so this basically kills the bot, you wont be able to just kick the dead ones out to add new alive ones):

bot_kick


Pretty fun for practicing the game when people aren't around to play or to make solo a bit easier.
Also a great help to obtain a couple of achievements easier.

All servers now allow you to vote on a lot of things by using the command "callvote"

If you press ESC there is already a menu option to use callvote with the UI.
But if you need the commands (for whatever reason) here is how to manually use them in console:


callvote changelevel "Level name"
Example: callvote changelevel cx_biotec

So you can start a vote to change the level to a specific one.



callvote ChangeDifficulty "Difficulty ID"
Example: callvote ChangeDifficulty 3

So you can increase or decrease the map difficulty (Needs a restart of the level)

callvote RestartGame

So you can vote to start again since it all went to hell.


callvote Kick <userID>

So you can kick a player trollong/griefing etc.