Develop your message. What is it you want the reader to learn from the books, in other words, how do you want to influence him? Write down some specific ideas, values or social issues that you want to address and include your own views on each topic. If any ideas come to mind on how to include them in the story, be sure to write them down as well.
Select a backdrop for your allegory by choosing a setting. The setting of the story will greatly affect all aspects of the story. A fantasy story gives you the ability to think out of the box and create a whole new world, rules that govern it and creatures that live in it. Fantasy is the most common genre that allegory exists in. Although, a simple battle of schoolyard children could be an allegorical representation of war, or even politics.
Develop a lead character. Good character development will greatly increase the interest of your readers. A good character will be one that the reader can identify with; the character will have strengths and weaknesses. Create a journey for the character. This can be an emotional journey, a physical journey or both. Readers want to follow a journey and see a character that is faced with battles within himself or against another. The lead character will come face to face with dilemmas that will introduce the allegorical issues contained in the story. You want the character to react and deals with these issues in a way that will influence the reader.
Develop supporting characters. Create characters that have different values that challenge the lead. Create characters that will be helpful throughout the journey. Make the characters complex and give them motives for what they do. Most importantly, each character must be true to his values. Use these characters to build up or tear down the issues you are writing about. Strong supporting characters will make the allegory more effective. Put the characters into the story and begin to develop it.
Write memorable moments into the story; these moments will influence, persuade and change the readers' mind on an issue. A memorable moment could be the final confrontation between two characters that leads to an illuminating choice. The choice made will be determined by how the lead character has grown or changed throughout his journey. There should be several memorable moments within the story, each needs to have a buildup to that moment and have a symbolic meaning in its events.