Decide how you're going to relate forensic science to your class. For instance, a biology class could cover how blood left at a crime scene can be analyzed to help identify the person who left it there at a later date. A physics class could discuss ballistics and how to reconstruct the path of a bullet based on evidence left behind at the scene. Even geologists can identify the soil and rocks left wedged in a tire's treads to see if they're indigenous to the area that the vehicle was found in, and if not, where they could have come from.
Outline the lesson. Just like a paper, you should outline the lesson to make sure you hit all of the important points before getting down to the details. Explain what forensic science is, how it relates to your field and give some examples of what forensic scientists do in your field. There should be an introduction, a body and a closing, just like an essay.
Fill in the details. Treat the lesson plan as if it was a speech that you were going to give to your class, and you want to make sure you have all the points down and all your grammar straight. Also, decide if you want to have an educational activity to give the class a hands-on feeling. As an example, an art class could have students pair up and have one student describe a person who committed a crime. This gives the students a chance to recreate a face like a police sketch artist. Alternatively, you could stage a crime scene and ask the students to collect evidence and based on what they find, recreate the crime.