Just as some publishers require a query, some publishers are adamant about accepting only complete manuscripts. That is especially true of publishers who publish mostly short pieces, material for children, humor, fiction, etc. They feel like it is easier to read a short manuscript than trying to judge it by a query.
When sending a complete manuscript you simply put your completed manuscript in an envelope with or without a cover letter. When determining whether or not to include a cover letter, check their listing in the market guide first to see if they ask for one. If so, include one that simply introduces you and your subject. If they don’t ask for one, you will still include one if there is something specific you need to tell them such as your qualifications for writing the piece (if the piece requires certain qualifications or experience), your reasons for using a pen name (if applicable), problems with permissions, or anything else it is important for them to know. (See Cover Letters in Part 3.)