Use numbers, names of equipment you've used, or features of the project that may apply to the job you want. One strong qualification, described so that the reader can picture you actively involved on the job, can be enough. You can then refer your reader to your resume for a summary of your other qualifications. If you have two or three areas that you think are strong, you can develop additional paragraphs. Make your letter strong enough to convince readers that your distinctive background qualifies you for the job but not so long that length will turn readers off.
Some employers recommend a maximum of four paragraphs.
Other Tips Refer to your resume. Be sure to refer to your enclosed resume at the most appropriate point in your letter, for example, in the discussion of your qualifications or in the closing paragraph. Conclude with a clear, courteous request to set up an interview, and suggest a procedure for doing so. The date and place for the interview should be convenient for the interviewer. However, you're welcome to suggest a range of dates and places convenient to you, especially if you travel at your own expense or have a restricted schedule.