It was in this course I was introduced to the poetry of William Blake. For those unfamiliar with Blake's poetry let me say, that's unfortunate. On the other hand, he is an acquired taste I think because much of his poetry revolves around the use of symbols, most so than other poets I think.
But I got the message and years later I have found symbols in my own work. For example, Evan Pierce, the main character in my first novel, as a priest, functions as the moral center of the story. Later, in Book 3 Evan has been written out of the story and the moral center is far more vague. I replaced Evan with Sir Ahlan, a knight from Evan's own order. While Ahlan is a great character he has a harder edge and the moral center takes on a more Old Testament feel.
Using symbols in writing is just one of many techniques available to the writer. But like a master chef cooking with a rare ingredient, you had best know what you are doing when you attempt to use a symbol or you'll ruin the story. Heavy-handedness with symbols is like playing a delicate musical phrase blaring and loud.
My recommendation is that you don't try to use symbols. Let them come to you. I never intended Evan to be the moral center of anything, but as I wrote my novels and got to know Evan, it suddenly occurred to me that he was a very honest and moral person; in fact, he was a good deal more ethnical than many others in his religious order. That sudden shock of realization happened in while writing Book 2 as I was writing about several of the priests. And when I realized that Evan was the more moral I used that knowledge in a subsequent revision of Book 1 to adjust Evan a little.
My other recommendation is to re-read your own work and look for any symbols you might already be using. Since the world is full of symbols, I'd be surprised if a few of them hadn't spilled over into your work. But if you can't find any, don't worry. They aren't required. They're just a nice surprise when they are there.