Everyone may have different steps that they take, but you will want to address a few key points. Some examples might be:
1) What do I want my students to learn?
2) What activities will I use?
3) How will I check for understanding?
Give the interviewer your structure for planning. For the activity, you may want to give an example such as a game, writing exercise, or discussion topics. Sometimes it's easier to discuss an actual lesson plan you taught in the past and explain how you came up with it. This is a question you want to be prepared with examples and details. They may ask you, "why" after you explain your process, so be prepared to talk about your method in depth.