Read fiction or drama in which symbols are used well. Make a list of all the symbols you note, the page numbers on which they appear and for what they seem to stand. After completing the work, review the list and review the page numbers you listed. Draft a few "rules of symbolism" you learn from this practice with each author. For instance, one rule might be, "Don't introduce too many symbols in one scene."
Write a first draft of your story or play without worrying about symbols. When the draft is complete, re-read it with an eye toward the symbols you may have naturally included. Highlight them. If you think these naturally occurring symbols re-enforce the theme of the story or augment the plot, enhance them by re-reading the text again and finding appropriate places to add more symbols.
Replace naturally occurring symbols that do not enhance the plot or theme. Re-read your manuscript for symbols that do not "work" with your plot or theme. For instance, you may have inadvertently used a cross for a symbol of suffering in your first draft, but you decide its religious connotation would be too confusing to the reader. In this case, choose a different symbol for suffering.
Create symbols when none naturally occur. Re-read your work again, this time taking note of what thematic or plot points you want to underscore with symbols. Then, you choose an appropriate symbol that your audience would easily understand. For example, you may want to underscore that the negative things that happen to your protagonist are due to fate. In this case, you might want to have him carry a pair of dice and finger those dice right before a negative event occurs. Insert these scenes into your manuscript and re-read them to be sure the tone is consistent with the other parts of the passage.