According to the Purdue Online Writing Lab, a modified block format for a letter is arranged in a standard block format except for the return address, date, closing and signature. These bits of information start at the center of the page. They are not left-justified as in block format.
For each of these sections, the Tab key is hit until it reaches the center, and then typing begins. At the top of the page, the street address is placed on the first line, and the city, state and zip code are on the next line. A space is skipped, and then the full date is included. At the bottom of the page, a closing, such as "Sincerely," begins at the center as well. The signature goes directly under the closing.