You might just have to take your lesson in a bit of a different direction than you thought, but that is OKAY! "We construct knowledge in a dynamic process through which we build upon prior knowledge as we experience the world." (Rethinking Curriculum in Art) As a teacher you need to be able to adjust, to make concessions and redirect. You might have to back up further in your lesson than you originally thought or you might need to expand it to something more complex. "We teach students, not subject"; that means your lesson won't be the exact same from class to class.
Students construct knowledge rather than receive knowledge. Because of this constructed knowledge, inquiry based instruction is crucial. Those enduring ideas, key concepts, and objectives are not just for you as the teacher. Your students need to know what it is they are supposed to gain from the lesson and how each piece of knowledge connects to what they are doing. Don't leave your students out of the loop and hope that they gain the information you want them to. TELL THEM what you want them to learn, over and over, and tell them how each step of your project reinforces this knowledge. Ask questions that get your students thinking about what they should be learning, reinforce those essential questions that you built your unit on.
We want our students to explore the subject or lesson through a series of questions, some of which can be provided but many of which need to come from the student. We want our students to be able to go off and learn on their own. In order to do this they need to understand what questions to ask and how to find answers.
Presenting experiences for the students to take what they have learned and apply it within other contexts is crucial for developing that new knowledge. Learning within the art room is all well and good but it needs to extend beyond and audience of peers and the teacher. Having students display their knowledge to an audience which might not be so well educated on the subject presents a challenge for the students that deepens their own knowledge and understanding of the subject. Some ways of doing this include: creating learning guides for another class, creating informational bulletin boards, or presenting a gallery with a demo.
Experiences such as these above, give the student an opportunity to demonstrate what they have learned in a more rounded way. Being able to create a guide for a class give students an opportunity to identify the most important aspects of a lesson and convey that to others. This is a far more in-depth understanding of the material than the ability to replicate and assigned artwork. Teaching someone else requires the student to ask themselves what questions and answers are crucial for understanding the material.