1) Start the map in Casual
2) Go to Settings --> Game --> Enable Developer Console --> Yes
3) Go to Settings --> Keyboard & Mouse --> UI Keys --> Toggle Console
4) Press the button for Toggle Console setting
5) Type the first command IN square brackets: [sv_cheats 1 ; sv_gravity 230 ; weapon_accuracy_nospread 1 ; mp_t_default_primary weapon_ssg08 ; mp_t_default_melee weapon_knife]
6) Type the second command IN square brackets: [mp_t_default_secondary "" ; mp_ct_default_primary weapon_ssg08 ; mp_ct_default_melee weapon_knife ; mp_ct_default_secondary "" ; mp_maxmoney 0 ; mp_restartgame 1]

Developer's console has a character limit, so you must type two commands in order.
You can do this basically with any map, such as Lake, Shoots

If you want to play the mode on the official remake of Shoots for example, then type the command in square brackets: [game_type 0 ; game_mode 0 ; map ar_shoots].
"ar_shoots" symbolizes the name of the map, that the game eats. "ar" means "Arms Race", "cs" means "Counter-Strike (Hostage Rescue)", "de" means "Defusal". "_" means the space between gamemode the map is made for and the map name, that the map eats. Normal space like you can see in "map ar_shoots" means the console command and then what to do, like "give weapon_ak47" the command give gives stuff to player, and "weapon_ak47" is the entity name or something like that. I hope you get it. And "shoots" means the name of the map. Though in CS:GO there was a Danger Zone map called "Jungle" with the name, that the game eats "junglety".

To play maps DOWNLOADED from the workshop if they don't have casual mode in it with this game mode, type [game_type 0 ; game_mode 0 ; map_workshop <id of the map> <map name game eats>].
"game_type" symbolizes the game mode the map is playing in, also the same for "game_mode", though they are working together to select the game mode from the grid. "map_workshop" command searches DOWNLOADED maps EXCLUSIVELY from the workshop. Replace "<id of the map>" with actual id of the map, which you can see in the url of the map (and this discussion). The code name of the map is trickier to find. You can though by typing for example "map_workshop 3070553020 ", the space helps you find the map name, that the game eats. Sometimes there might be more than one, being for example like the skybox map.

If you need more info about the gamemodes, then here's a link: https://developer.valvesoftware.com/wiki/Counter-Strike:_Global_Offensive/Game_Modes .
Remember that some of them might not work just yet, like Danger Zone. You can play Training Day and Demolition (though Demolition is WIP).

If you for some reason wanna launch this map via console in Casual (even though it already has the gamemode implemented) then here's a command: [map_workshop 3070553020 ar_dizzy].

Sorry for the nerd info