Label experimental variables on them. Develop the mathematics of your experiment. Derivations which start with fundamental physics are appropriate. At some point you should have a clear concise outline of your procedure. Tell the reader what you did and why you did it.
Speculation is good.
Make notes to yourself of things to try. Each student will submit their own report, even though you may do the experiment as a group. Reports are expected to meet the highest editorial and scientific standards. Pretend it is being submitted to a refereed journal for publication and that your job is on the line. The details of your format are dictated by those used in a journal you choose which is consistent with your career goals.