1. In order to foil a terrorist plot, you must first find a terrorist plot. This is not easy.
2. Not just anyone can find and then foil a terrorist plot. You must have an incentive. The best incentive is to be an accused felon, looking at a long prison term. Under such circumstances, your lawyer will explain to you, you may be able to reduce your sentence by acting as an informant in a criminal case, preferably one involving terrorists.
3. The fact that you do not know any actual terrorists should not in any way deter you. Necessity is the mother of invention: if you can find the right raw material -- a sad, sick, lonely, drunk, deranged, disgruntled or just plain anti-American Muslim somewhere in the United States -- you can make your very own terrorist.
4. Now the good part begins. Money! The FBI will give you lots of money to take your very own terrorist out to lots of dinners where you, wearing a wire, can record yourself making recommendations to him about possible targets and weapons that might be used in the impending terrorist attack that your very own terrorist is going to mastermind, with your help. It will even buy you a computer so you can go to Google Earth in order to show your very own terrorist a "top secret" aerial image of the target you have suggested.
5. More money!! The FBI will give you even more money to travel to foreign countries with your very own terrorist, and it will make suggestions about terrorist groups you can meet while in said foreign countries.
6. Months and even years will pass in this fashion, while you essentially get the FBI to pay for everything you do. (Incidentally, be sure your lawyer negotiates your expense account well in advance, or you may be forced -- as the informant was in the Buffalo terrorist case -- to protest your inadequate remuneration by setting yourself on fire in front of the White House.)
7. At a certain point, something will go wrong. You may have trouble recruiting other people to collaborate with your very own terrorist, who is, as you yourself know, just an ordinary guy in a really bad mood. Or, alternatively, the terrorist cell you have carefully cobbled together may malfunction and fail to move forward -- probably as a result of sheer incompetence or of simply not having been genuinely serious about the acts of terrorism you were urging it to commit. At this point, you may worry that the FBI is going to realize that there isn't much of a terrorist plot going on here at all, just a case of entrapment. Do not despair: the FBI is way ahead of you. The FBI knows perfectly well what's going on. The FBI has as much at stake as you do. So before it can be obvious to the world that there's no case, the FBI will arrest your very own terrorist, hold a press conference and announce that a huge terrorist plot has been foiled. It will of course be forced to admit that this plot did not proceed beyond the pre-planning stage, that no actual weapons or money were involved, and that the plot itself was "not technically feasible," but that will not stop the story from becoming a front-page episode all over America and, within hours, boilerplate for all the Republican politicians who believe that you need to arrest a "homegrown" terrorist now and then to justify the continuing war in Iraq. Everyone will be happy, except for the schmuck you shmikeled into becoming a terrorist, and no one really cares about him anyway.
So congratulations. You have foiled a terrorist plot. Way to go.