Familiarize yourself with the material in more detail than you intend to teach with; this level of preparation will enhance your own understanding of the subject and help you answer any in-depth questions your students may ask.
Decide how much detail you want to provide on conduction, convection and radiation and construct a rough outline of your lesson plan. You want your students to understand the processes and the reasons why they occur, but you don't want to overload them with scientific terms and excess information that could interfere with learning and make them lose interest.
Include examples. If students can relate these processes to real life events that they have witnessed, they can usually grasp them quicker and with more confidence. For example, an example of conduction would be holding an ice cube and feeling it cool down your hand. Convection can easily be observed in a boiling pot of water. And radiation is the culprit behind sunburns.
Outline at least one alternative explanation for each process. Some students may have trouble understanding how the process works through your first explanation.
Design homework assignments and exam questions according to your lesson. Don't include questions that students cannot answer based on the information you taught.