SFTP for 3rd party to access a directory

I have been trying, without success, to create an SFTP account for a third party to have access to a single directory. I was successful with plain FTP, but can't find instructions specific to my situation. For example, one of the InMotion instructions I read talked about using the cPanel username and password. But I don't want the 3rd party to have cPanel access, I only want them to see one directory (as I was able to do for plain FTP). I also am unclear as to what I did to give to the 3rd party for them to use an SFTP connection -- do I give them a private key, the public key, what format, etc.