The interviewer wants to learn your process of lesson planning. 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.
"I feel that the most important step to developing a lesson plan is first to set my learning objectives. Then, write a very clear outline. The final step would be for me to set learning timelines and deadlines for each."
"When I am developing my lesson plan, I first assess what the learning goals are in the classroom as a whole, and then I assess per student. Once that is figured out, I plan my timeline to ensure deadlines are met, and lessons don't go over time."