Open the console (shift and ~), type \addbot <name> <skill> <team> <0> <funname> where
<name> is the official bot name like jan, luke, kyle, tavion, etc.
<skill> is a number 1 to 5, 5 being the highest skill
<team> red or blue
<0> number that does nothing, I use 0
<funname> name that the bot will have in the game like "n00b"
only name is required, the rest are optional. So you could use
\addbot luke 5
\addbot luke 5 BLUE
\addbot luke 5 BLUE 0 "n00b"
If you are controlling a remote server, use \rcon addbot