There are exactly two ways: 1) The way your teacher or boss wants you to. Please ask them for a sample and follow it. They didn't get to be the boss by having any original thoughts. Neither should you. 2) Write the lab report so that ten years from now you can reproduce the experiment. You want to answer the question, "Exactly how did we do this? What were the conditions, measuring devices, ambients, procedure, set-up, controls, date, personnel, conclusions, etc....?" Don't leave data, photos, graphs, etc. on some computer that may not exist later; include them. Don't depend on the memories of participants, document them, too. Sometimes this can be extremely simple. Sometimes thousands of pages are required.
A lab write up is something that is done after a student does an experiment in a laboratory. The write up should follow a very strict set of guidelines, and should have an introduction, question, body, and conclusion.