You should include a description of the theory or principle to be tested, including any relevant equations, and the method you will use to test this theory. Mention the measurements you will make, describe how you will reduce your data, and state what the theory or principle under consideration predicts the result of your data analysis to be. You should not simply list your tools and equipment, but rather explain in one or two sentences what equipment you used. You should also mention the manufacturer and model of your equipment, where appropriate. You should provide enough detail that an interested student who does not have access to the lab notes can reproduce your work.
Diagrams are usually very helpful.
You should also show all your data, and they should be in neat, printed tables. If appropriate, your results should also be contained in a table or tables. However, it is not necessary to show all your calculations, or even to show a sample calculation. Describe the results of your investigation. Compare the results you obtained to those predicted in the theory section.