Since newsletters are often published by volunteers, they probably cannot pay you. This letter is essentially a one-page proposal describing your idea, your qualifications, and the reasons they should carry the article. Your library has books on writing query letters. Keep in mind that editors are not necessarily industry experts. This means you may have to do some explaining to convince them that your idea is worth publishing.
Editors have deadlines.
If you are late, you probably will not get any more assignments by that publication. As you write, remember that the article is not an ad for your services. Write an informing article that is consistent with the publication's theme. Have a trusted colleague or professional writer review your work for clarity, content, and organization. After your article is published, make copies to use with your marketing literature.