An allegory presents a complex truth about the human experience, usually pertaining to moral or political implications. In order to craft an allegory, you'll need to rely on several literary techniques.

Theme and Purpose: You'll first need to begin with the theme or truth that you want your allegory to expose. Perhaps you will choose to examine the dangers of a particular political era or ideology. Or perhaps you want to demonstrate humanity's need to constantly embrace technology meant to make life easier—regardless of the cost to the planet and/or natural resources. Maybe you want to examine the struggle individuals face if they choose to forge a true path of unique self-identity. Many of Aesop's fables can be read as allegories, so you might want to read (or reread) some of those to get your creativity flowing.

Characterization: Once you determine your allegorical purpose, you'll want to choose characters who intentionally represent the conflict inherent in your purpose. To keep things simple, you may want to write with simply a protagonist and an antagonist. As these two characters engage in conflict, they should propel the purpose of your allegory.

Symbolism: Allegories rely on the use of symbolism to create meaning. Consider the Truffula trunks from The Lorax by Dr. Seuss. Although the needed tuft can be harvested without cutting down the trees, soon mechanization arrives with the ability to chop down four trees at once, wasting a valuable natural resource. This comes to symbolize the greed of human mechanization to the detriment of nature. Including pointed symbols in your allegory helps to construct your intended theme.

Allegories don't have to be long to achieve the intended purpose. You might find it simpler to keep the plot simple as you focus on a limited number of characters and utilize a couple of key symbols that relate to your overall purpose.