Use the phrase c/o, or care of, to address a letter sent in the care of another person. Write the recipient's name on the first line, then write c/o and the name of the person who is responsible for conveying that letter to the recipient on the second line.
C/o should be used when sending mail to someone who is not a resident at an address or who does not have a standard address. It specifies the resident who is responsible for delivering mail to the final recipient. Care of can also be abbreviated as c/-. According to MacMillan Dictionary, this abbreviation is most common in Australia and New Zealand.