Whether the situation is a standardized test like the GMAT, a classroom setting or any other setting, the most important rule to follow is reading the directions carefully. The sentence changes will differ based on the directions and the type of changes the directions dictate. For example, directions might dictate standard English, which is grammatically correct and less formal, but does not require editing for redundancy. If directions dictate correcting for clarity and precise meaning, then check for grammar and redundancy.
Check whether the subject and verb tenses match. On tests or when sentence correction is necessary, one of the easiest problems to spot is tense differences. The subject and verb must agree. So, a plural subject needs a plural verb to match; likewise, a singular subject needs a singular verb. Matching the tenses and forms of the subject and verb is one of the main rules of correcting sentences.
Main grammar errors, like using "it's" instead of "its" or placement of apostrophes in the wrong location, need changes. The grammar error of mixing up "it's" and "its" takes only a moment of thought and checking the surrounding sentence. If the sentence reads properly as "it is" then the appropriate word is "it's"; if it is a possessive, then the appropriate word is "its." Change the grammar errors so the meaning of the sentence remains the same, but the sentence is grammatically correct, with the appropriate agreement between the subject and the verb.
Reading out loud will make it easier to locate mistakes. Grammar mistakes, wording mistakes or improper usage of a word are easy to hear when it is said, but not always as easy to spot when reading silently. Note the pauses, which indicate the use of a comma, period, colon or semicolon. Note any odd-sounding spots and look for the word or words that make the part of a sentence sound weird. Make changes until it sounds correct and flows when read out loud.