Having spelling errors on your report makes it look amateurish. Read every word carefully, and if you think a word looks odd, look it up. Don't rely solely on your word processing program's spell check feature either, especially if you are writing a technical report. Pay extra attention to homophones like "they're," "their" and "there." Watch for repeated words as well. These types of errors are very easy to miss.
Read each sentence carefully when doing a final proofreading of your paper. Make sure each sentence is a full sentence with a subject and a predicate. Remember that dependent clauses are not full sentences. Split run-on sentences into multiple sentences, or use correct punctuation such as a semicolon or a coordinating conjunction preceded by a comma. Look at each piece of punctuation that you use and make sure that you are using it correctly. Improper use of punctuation can damage the clarity of your sentences. Again, word processing programs often have trouble with more complex sentences, so don't change something just because the program indicates you should.
You also need to check basic grammar in addition to sentence structure. Make sure each of your subjects and verbs agrees in number. Do not have a plural noun with a singular verb or vice versa. Search for any lists or series of items that are not parallel and fix them. Read your paper aloud, and pay attention to any odd sounding sections. Rewrite these sections until they flow smoothly. Pronouns and antecedents must agree in number.
Check each of your citations to make sure they are correctly formatted. If you are not sure where you found a piece of information, look through your sources until you find it. Unintentional plagiarism is one of the worst mistakes that you can make in a report.
Literally look over your printed report before turning it in. If any words or paragraphs physically look different than the others, return to your report file and fix the formatting. Pay attention to each of your heads and the spaces between paragraphs when checking formatting.