For the citizenship renunciation to be legal, you must do the following:
Appear (in person) before a U.S. consular or diplomatic officer.
Further, it must done IN a foreign country ... So a good way to do this is at a U.S embassy or consulate's office.
And you must sign an oath of renunciation.
And finally, you should have some help you (at the embassy, they may be able to direct you to someone) to get the past tax returns done and see if you owe anything.