Main Concepts: Here are some general guidelines/best practices:
Using the teacher as a model. Literally, you as the teacher should draw a lot, as well as do lessons as directed drawings.
Look deep. Talk about careful observations and seeing details.
Talk with an art teacher at your school if you have one. They might be able to give you common language to use, or even help flesh out lessons.
Plan to have a lot of lessons outside AND collect many various specimens that you can bring into the classroom for poor weather days, short lesson blocks, or lecture-intensive days. Drawings can also be made from quality pictures or 3D models (see Resources for a few suggestions).