This gadget I'm handing you is called an override module! Mr. House had two of these made years and years ago - just like him to think ahead! Take the module to the El Dorado substation and attach it to the power control terminal! I'll handle the rest!
You will be given this quest once you finished all Wild Card quests except this one 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. Unless you are idolized by the followers, she will decline your request mentioning a short supply and limited resources. You can obtain idolized status by donating medical supplies, leave, and return until you are given idolized status and offered to join the Followers. Once you're done, report back to the Lucky 38 and complete your quest. Note: Due to a scripting error, this optional objective will only become available after you have completed Friend of the Followers.
Unless you are wearing NCR faction armor and so act in disguise, Boone will leave your party if he is with you when installing 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.
Doing the optional quest of asking Julie Farkas to support an independent New Vegas has no effect on the Old Mormon Fort ending, although the Followers will leave Medical supply boxes for the Battle of Hoover Dam.
A cut stage of the quest required the player to meet a Securitron near the El Dorado Substation after planting the override chip. The Securitron would lead you to a nearby ridge facing New Vegas, and a cutscene would activate showing the Lucky 38 being lit up.