From Halopedia, the Halo wiki
To see a Hitchhiking Flood, you must play the level Quarantine Zone on Halo 2. Get a Ghost and drive it through the level until you get to the big door (just before reaching the Gondola) where two Flood Combat Forms come out of. Ram one back into where it came from and let it board your Ghost. If the door closed behind you, with a little luck, the Combat Form will 'freeze'. The Combat Form will stay on your Ghost in attacking position, but it will not attack you, and you can drive around with it on your Ghost.
If you exit the Ghost, the Combat Form will still remain attached and the only way to get back on it is to kill the Combat Form.