Write down the details you already know about the class. Include age group, number and skill level of students and any special requirements -- for example whether it is a business English class, a conversation class or a grammar class
Write down what you aim to teach in the lesson, including grammatical structures, vocabulary and practical skills such as asking for directions. In most classes you could include one or two grammatical structures, one practical skill, and relevant vocabulary. This is your goal for the class.
Write down all the steps, in order, you plan to go through in order to accomplish your goal. For example; Step One -- introduce new vocabulary with pictures by brainstorming with students what these pictures are of. Include all activities, games and teacher explanations.
Note all the materials you will need such as handouts, board games, a ball, pictures, a map of the area and realia -- for example train tickets and so on. Include details of any coursebooks and a note of the pages you plan to use.