Report writing is one of those things that the more you do it, the better you get at it. First of all, the normal rules of grammar, punctuation and spelling apply. Additionally, try to avoid the use of contractions. "I could not see. . ." rather than "I couldn't see. . .", "They did not suggest. . ." rather than "They didn't suggest . . ." Write in complete sentences and try to avoid run on sentences. (It may help to think you are writing about the experience to a 10 year old niece or nephew, so keep it plain and simple.) As Mary mentioned, opinion is never part of the report unless it is specifically requested. And most companies provide you a sample narrative that can give you a really good idea of how much detail they seek. On most reports you need to answer every "no" question in your narrative, no matter how long the narrative gets. Some also want supporting comments for "yes" answers, so read through their instructions.
Good luck with it and don't let it scare you.