Just go Play -> Start Game. Select the options you want, remember that whatever you set the max players as will affect how many bots you can play. If you set it at 8, that's 7 bots etc.
Then, once in game, type 'add bot 'name'' in the console. Eg 'addbot Rickers' will add a bot called Rickers. If you find that 4v4 etc is too easy, you can use the 'forcejoin # colour' command. This command forces the player with the ID # to join the colour you choose. Eg 'forcejoin 5 red'.
Note:
-Console is what pops up when you press ~ (above TAB)
-To find out player ID's, type in 'players' in console.
Hope I have helped. Goodluck with Digital Paint: Paintball 2!