Each paragraph should express and develop one idea with adequate support and without contradiction, irrelevancy or redundancy. Check to see whether your paragraph clearly states the key idea it is built around. Most of the time a paragraph will start or end with one sentence, called a topic sentence or thesis statement, that states the paragraph's topic explicitly. Cut out any sentence that does not relate to the point of the paragraph. If a sentence is relevant to the paper overall but not to that paragraph, consider moving it elsewhere.
Well-written paragraphs must have no errors of spelling or grammar. As a first step, run the paragraph through your word processor's spelling and grammar checker, but be sure to do a manual check as well. Automatic spelling and grammar checks are unable to find all the errors that may exist in a paragraph. As you edit, check for consistency of tense and number, misuse of homonyms and correct punctuation and capitalization. When in doubt, check a reference work.
Make sure your paragraph does not use unnecessary or confusing words. Each sentence and each word should add new, useful information. Read the paragraph out loud or let someone else read it in order to make sure that the ideas flow clearly and the wording of your sentences is not confusing. If you find ambiguities in the phrasing, re-word the sentence or phrase in which you find them until your intended meaning comes through clearly.
Adequate research and error-checking also are very important parts of writing a good paragraph. Find a reliable reference that you can use to verify the assertions you make in your paragraph, including any figures or statistics you cite. A reliable source is one that states where it got its information or is widely acknowledged as a trusted expert in the relevant field. Search your paragraph for any points about which you are uncertain and do whatever additional research is necessary to adequately verify, modify or delete them.