Step 2: Get an especially concentrated form of amonia, as a "cleaning" agent, and use it to render unconcious the entire staff of the building (be sure to wear a gas mask while doing so, lest you become prey to your own malicious plot)
Step 3: Alter a few of the pre-recorded TV shows, and commercials, inserting subliminal advertising.
Step 4: Run for the office of World Dictator, and use political contributions to buy off several more janitors, promising them positions of power as well.
Step 5: Use your espionage oriented janitors to insert a great many more subliminal advertisements, that will bring a warm fuzzy feeling when they vote for you.
Step 6: Hold the election, and win it. (if you fail this step, simply repeat steps 1-5)
Step 7: Your the world dictator! Be sure to abide by the rules for evil overlords, and crush all who oppose you (without explaining any evil plots of yours to them)