During The Game, but first the code has to be activated by making a shortcut to enable opening the in game console
First Make a new short cut on your desktop and have the target leading to HL.exe i.e (D:\\Sierra\\Half-Life\\hl.exe)
DONT PRESS ENTER YET! Now on the end of this target adress type two space then type \"-dev\" and a another two spaces and then \"-console\" This should look like
D:\\Sierra\\Half-Life\\hl.exe -dev -console
Your hard drive might not be D so change it, it might be best to copy and paste that directly.
After you have created the new shortcut double click and use this to open half-life. Select your custom game, play/load a game as normal (not on internet unless enabled by server) and press the TLIDE key during play (the key left of the 1 key (`)) This will open up the console simply type in the following available cheats.
(god cheat enabled)
(fly around trough walls)
(enemies will ignore you but still kill the scientists and do their dirty deeds as usual)
(two spaces between that - enables all weapons and ammo and armour, wont need that if your in god mode!)
There are other MORE complicated cheats such as loading specific maps.
When you save the game the cheats opened will be enabled as you load.