Step One: Don't be an idiot
Now, I don't mean to offend anyone here (but there's a few people I hope it offends anyway). I'm not talking about having a high IQ, doing well in school, or how many Nobel prizes you keep in your closet. All I want to get across with this step is one simple idea: If you think it is a bad idea, don't do it.
I have seven. Four of them are made of chocolate.
Step Two: Never brag about being good at math
Now this one may seem counter-productive, but here me out. How many times have you had this conversation in your life:
Other Person: Dude, I am so good at math, I can do vector calculus in my head backwards while playing boxman on my calculator!
You: Good heavens, young man! Your god-like mathematical prowess and nonchalance about a topic most people have nightmares about has so thoroughly impressed me that I have founded and now preside over your fanclub! Please sign my face and have my babies!
You do not want to know what they do with this thing in math nightmares...
That never happens. In fact, a far more likely response to hearing that someone is good at math is to not believe them and ask them to prove. And that would blow your whole plan out of the water, Mr.Pretending-to-be-good-at-math (The fact that you're reading this article proves to me that you failed algebra twice because when they asked you to find x you circled it and told them it was way harder to find Waldo). In fact, someone bragging is ten times more likely to make someone think you're lying than it is to impress anyone. It's a scientific fact, one I just made up, but a fact nonetheless.
Step Three: Don't Panic
Not to steal an idea for A Hitchhikers Guide, but this step is still important. Math phobias are extremely common in our modern society, and are a sure sign of a bad case of the sucksatmaths (luckily it isn't contagious). Regardless of how terrifying a bunch of numbers on a page can be, staying calm in the face of impending numerical and algebraic doom will easily convince any passer-by that you know what you're doing.
Don't scream. Don't even blink. The cosines can smell fear...
It turns out, an important part of looking like you're good at math, actually involves what you look like (go figure). However, this step really needs to be broken down into two separate parts, depending on what kind of math you want to look like you know. If you want to look like you're a whiz at arithmetic and can crunch numbers faster or more accurately than the rest of your mathematically challenged social circle, see part A. If you want to look like you understand multivariable calculus, number theory, and/or anything your half-insane Romanian math professor has been shouting at you all semester, see part B. Now that I'm almost positive everyone is going to read both parts, we move on...
Part A: This method calls for very little alteration to your actual looks, which is very convenient for those of you that might want to look like you're capable of things other than math (for example, socially acceptable personal hygiene habits). Firstly, don't wear math paraphernalia; t-shirts, hats, pocket-protectors, designer protractors, none of it. This will attract people actually capable of mathematics, and suddenly you're in the wrong social circle for this guide.
The single most important thing to do for this part of step five is to convince all of your friends that you have picked up a horrible addicting habit, namely constantly texting on your phone.
Part B: This part here is a little more complicated than part A, so pay close attention. In order to create the appearance of someone capable of higher level math, you might have to undergo some drastic changes. First, give up personal hygiene entirely, one bath a month will probably do, and stay as far away as possible from deodorant. Second, lose those social skills, panic at the site of strangers, tell awkward jokes that even you don't get, and if possible develop a twitch (mine was a near constant blinking, aside from all the things I ran into, only seeing half of what was going on was totally worth it!). Finally, if you still have any friends willing to talk to you, cut all ties with them except for the internet. Use the remaining internet connections to send people links about the strangest things you can find on the internet, preferably those about robots, the Discovery channel, or anything that ever happens in Japan.
Or, if all of that seems like more work than actually learning something, you could always read this. And then quote it extensively in casual conversation.