I made a post about it earlier, and that I wasn't too sure on how to do it. Well after trying a few more times and playing around, I figured out how exactly to do it. On the last level, Halo, near the end after fighting that militia of flood to get to the control room, in that hallway there is that last door you go through which ignitions a cut scene.

Well, first you want to do the "Infinite Spartan Lasers" glitch (this one is optional). Kill Johnson, take his spartan laser, but make sure that you throw the gun far away so he gets back up with a new spartan laser, not picking up the gun you just dropped (sometimes the first laser doesn't have infinite ammo). Once you have a laser with infinite ammo, drop it somewhere in that hallway, then kill Johnson again to get a new Spartan laser with infinite ammo (because after that easy fight, you have the assualt rifle again, and nothing else, but one of the infinite lasers will be there with infinite ammo which you can use till the end of the level).

Now, to get Sergeant Johnson still with you, stand by that last door that ignitions the cutscene, but not too close to where it will start that cutscene. Now aim at Johnson and kill him, after doing so, immediately go through that last door before he even starts to revive. Then after that easy fight, you will still have him with you, and if you let the infection forms kill him, one of them will go into his body, thus turning him into flood.

Here's a fileshare for those who would rather see a video on how to do it, and sorry I haven't gotten it uploaded because my capture card isn't working (linked 3 times just in case of missing letters)

http://www.bungie.net/stats/Halo3/FileShare.asp ?gamertag=Triptych229

http://www.bungie.net/stats/Halo3/ ileShare.aspx?gamertag=Triptych229

http://www.bungie.net stats/Halo3/FileShare.aspx?gamertag=Triptych229