You will be given this quest once you finished the Wild Card: Side Bets and Wild Card: You and What Army? quest from Yes Man. It requires you to upload an override chip into the El Dorado substation controlled by the NCR. An optional objective to the quest is for you to save President Kimball when he arrives to give his speech. This option is only available if your reputation with the NCR is in good standing, otherwise you cannot accept the quest from Yes Man.
The main part of this mission is very simple. Go to the El Dorado substation and upload the override chip into the terminal located inside. There will also be a few NCR troops inside though that will shoot on sight so try to sneak in with a Stealth Boy if possible. The best way is to wear NCR armor, the guards don't attack you that way. If you did the optional part of this quest and kill the troops in the substation it will not matter to your reputation because you are well liked. Another way to avoid being attacked by the NCR troops in the substation is to immediately exit it once you've entered it. When you then enter it a second time, the NCR troops will not attack you.
Optional: Yes Man asks you to convince the Followers of the Apocalypse to give full assistance to the New Vegas region; simply go and talk to Julie Farkas. There appears to be no way that she'll decline, so once you're done just report back to the Lucky 38 and complete your quest.
Go to the El Dorado power station and install the Override chip.
(Optional) Convince the Followers of the Apocalypse to support an Independent Vegas.
Report back to Yes Man.
Unless you are wearing NCR faction armor and so act in disguise, Boone will leave your party if he is with you when install the override chip at the substation. Without the benefit of a faction disguise, a work-around is to tell Boone to wait outside of the substation so that he does not witness the chip installation, then he will remain with your party.
If you've done Boone's quest beforehand he doesn't seem to leave as long as you don't kill anyone.
Even if the player is already 'Idolized' with the NCR, you will lose the quest You'll Know It When It Happens if you go to the El Dorado substation first. Completing You'll Know When It Happens first and then going to the substation results in the Quest ending with your NCR fame at 'neutral' or higher. On at least one playthrough on the 360, completing this quest after You'll Know It When It Happens left NCR reputation at 'Idolized.'