The first thing you have to do is get some connections on the political end. Pick your political party, but it's gotta be the Republicans or Democrats (this probably won't work for Ross Perot). Second, get elected. The easiest thing to do, and the way we'll go about it to make it fastest, is to become Vice President. One can go straight from the House to this, unlike the President, who is almost always a governor or senator (unless Vice President first). So you've been elected to the House first. Then, you get on some prestigious committees, particularly those with a partisan bent (any, but especially Rules and/or Ways and Means). Make your name here by sponsoring some legislation that might be controversial, but which the party leadership is stuck on. Also, do grunt work. This often appeals to party leadership. Remember, Nixon was only a congressman before he became Vice President under Ike. How did he get the job? HUAC. (actually, Nixon was a senator for two years, but most people know him as a congressman) Also, utilize the media as much as possible. This gives you exposure, which can be manipulated to make yourself look favorable and give you an edge over other vice-presidential candidates. Finally, make ABSOLUTELY SURE that you are very good friends with the leadership in the Senate and House. Even if they're in other parties, buddy up to them, you'll need them later.
Let's suppose you're able to get the vice-presidential nod. Coming in as just a congressman, you're not going to be able to demand much. But either try to get allies appointed as Cabinet nominees if you're elected, or ally yourself with those who will be Cabinet secretaries. The most important ones? State, Defense, Treasury. Now, you need to get elected. If you don't, you're out of luck until you can raise enough money to hire Executive Outcomes to foment a coup.
Another supposition: let's say you get elected. Here I have to give credit to two things for this part of the plot: first to the book Father's Day, which details a plot using the 25th Amendment and the Presidential Succession Act of 1947 to take over the presidency. Second, I have to thank Mr. Baskette, my high school government teacher, for showing me how important the Constitution is.
Once in office, the wheels begin to turn. If you've got the president pro tem of the Senate, speaker of the House, and most importantly, a strong majority of the Cabinet (esp. the aforementioned State, Treasury and Defense Departments), you can use the fourth section of the 25th Amendment. And I quote: "Whenever the Vice President and a majority of either the principal officers of the executive departments or of such a body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Spreaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President."
Pretty slick, huh? And it's all legal, since 1967, when the 25th Amendment was ratified. And to think, something written in all likelihood because of fear that Kennedy could have survived his shooting and been incapacitated (who would have been president, constitutionally?) can be used like this. Another example of how this could have been beneficial in the past was when President Wilson suffered an incapacitating stroke in the end of his presidency but "continued" as president (his wife really ran things). Yeah, you can look it up, it's true. Also, the courts can't do anything about this. Since it's an amendment to the Constitution, they can't judge it unconstitutional. Hah! By the way, you might want to "influence" the president pro tem by offering him the post of vice president. Although the Speaker is closer in line, the president pro tem might want more power. However, either the p.p.tem or the Speaker would be bribable by offering a v.p. slot.
Now there is a catch here. It would be a lot easier if the President would either resign, or submit himself that he was unfit to serve. However, we have to assume that he did neither of these. There is a clause in section 4 stating that the President can go to the President pro tem and the Speaker and say that he is fit. However, one way to prevent this is if your connections to the Speaker and President pro tem are tight, they can simply deny that they ever were in contact with the ousted president. However, this might not work. In this case, you have to use a "challenge." In this, you do the same thing as when you declare the president incompetent, but you have to have a 2/3 vote in both houses to maintain power. Otherwise, the guy (or girl, possibly) that you deposed gets their power back.
The next steps of the plan, namely maintaining power, depend on one thing: we have to assume that the Acting President maintains all the powers of the elected president. If so, then one of the first things to do is to consider some patronage for members of both houses of Congress. This might appease some of them for a time, while you deal with another branch of government: the judiciary. First, with the appeased Congress, start impeaching district court and circuit court of appeals judges. Eliminate any who might be seriously detrimental to your plans, and replace them with allies. One excellent bet for district court judges is to employ senatorial courtesy, and use this as a patronage tool to boost yourself in the Senate. Also, remember, often staff members for members of Congress are lawyers. Circuit court judges! Now, with the Supreme Court, what you've got to do is go with FDR: court-pack. This, again, requires Congress's help, but promise jobs. My recommendation is to raise the number to 15; odds are that two or three justices might help you, and 15 would give you an automatic majority for what you need. This takes care of the judiciary.
In divying up spheres of influence, I would make domestic policy the confines of Congress and the President himself, and foreign policy the domain of the National Security Council, and the House and Senate Armed Services and Foreign Relations Committees. Also, I'd dissolve the Senate Select Committee on Intelligence, this could get a little nasty if they start investigating. You have to come up with the policy, but remember that you can't dissolve Congress (unless you REALLY have the Supreme Court in the palm of your hand, or you don't care about public opinion), and like it or not, elections are every 4 years, with a constitutional amendment allowing only 2 terms. The only luck you have is to have a national state of emergency (such as a war, depression, etc.), to say that you have to go against the law, or a constitutional amendment to allow more than 2 terms and change the length of terms.
Remember through all this, the most important thing is to keep it looking good for the public. Your fascist reign of terror shouldn't start until your base is solid. But watch your back. Because the one who takes power is the most likely to lose it.