Being conventional is the same as being traditional: a conformist. If one conforms to the general mores, ideologies, and thoughts of a particular time and place, one conforms without thinking, without questioning things, and without listening to one's own inner voice. Early in his essay "Self-Reliance," Emerson adds:
Whoso would be a man must be a nonconformist. He who would gather immortal palms (honors, awards) must not be hindered by the name of goodness, but must explore if it be goodness. Nothing is at last sacred but the integrity of our own mind. Absolve you to yourself, and you shall have the suffrage of the world.
In other words, to be an individual, one must not conform. If you are to become a legendary thinker ("immortal palms"), you must explore concepts. Be faithful to your own mind and then you will have the admiration (support or votes - "suffrage") of the world.
Later in the essay, Emerson gives particular examples. Rather than offer up prayers to a "foreign addition" or some foreign virtue," the original thinker will help himself. Emerson also advises that to avoid being conventional, one should never imitate. And this can apply to anything from modes of thinking to how you decorate your house.
Although some conventions change, Emerson is skeptical of social progress. Therefore, he would advise that to be original, one must not simply ride the supposed wave of social progress:
Society is a wave. The wave moves onward, but the water of which it is composed does not.
In other words, time moves on but the water (society) does not. Emerson believes that with each progressive advance in society there is a corresponding regression. (Think of how technology has helped us but has made us less inclined to think for ourselves.) So, Emerson would say that the conventional person is one who simply goes along with the ideas of society's ebbs and flows. But the original thinker looks to himself for ideas, does not just follow the trends mindlessly. For example, in 2015, is it conventional or original to spend 3 hours a day on a cell phone? Is it conventional or original to ascribe to one of two dominant political parties?