Open the console (by pressing ` or any other key you have bound the console to) and copy & paste the following into it all at once.

sv_cheats 1;
mp_limitteams 0;
mp_autoteambalance 0;
mp_roundtime 60;
mp_roundtime_defuse 60;
mp_maxmoney 60000;
mp_startmoney 60000;
mp_freezetime 0;
mp_buytime 9999;
mp_buy_anywhere 1;
sv_infinite_ammo 1;
bot_kick;
sv_grenade_trajectory 1;
sv_grenade_trajectory_time 10;
sv_showimpacts 1;
sv_showimpacts_time 10;
mp_solid_teammates 1;
mp_restartgame 1;

Start a private casual match with bots and join the Terrorist team. This will setup the game so that the bots will charge at you with knives out, helping you practice flick shots, reaction time and much more.

sv_cheats 1;
mp_limitteams 0;
mp_autoteambalance 0;
mp_roundtime 60;
mp_roundtime_defuse 60;
mp_maxmoney 60000;
mp_startmoney 60000;
mp_freezetime 0;
mp_buytime 9999;
mp_buy_anywhere 1;
sv_infinite_ammo 2;
bot_kick;
bot_add_ct;
bot_add_ct;
bot_add_ct;
bot_add_ct;
bot_add_ct;
bot_add_ct;
bot_add_ct;
bot_add_ct;
bot_knives_only 1;
mp_restartgame 1;

To change the map in your private match or server, simply enter map followed by the map you wish to use.

Examples:

map de_inferno
map de_nuke
map cs_italy
map cs_assault

de is used for Bomb Defusal maps, and cs is used for Hostage Rescue maps.

Once You have copied and pasted all of the commands in (and/or entered your own), copy the following to restart your match with the commands entered above:

mp_restartgame 1;

In most cases that line will already be added to a list of commands but it doesn't hurt to type it in again if the commands don't work first time around.

Thanks for taking time the to read this!
After entering these commands a few times it will become easier to remember them and you'll be an expert in no time, like this guy -