Since CS 1.2 was a , you need two things: a clean installation of Half-Life (original, not Source) and the CS 1.2 mod files.
However, pre-packaged "CS 1.2 standalone" builds exist, which include a cracked Half-Life engine. Download Counter Strike 1.2
Finding an "official" standalone download for CS 1.2 today is challenging because modern platforms like automatically update the game to the final version, The Non-Steam Method: Since CS 1