If I understand the question correctly you are looking for some help on implementing your own implementation of the FTP protocol using SFML's sockets? If so I would say a good starting place would be to take a browse through SFML's own implementation of the FTP protocol. It should give you a good outline as to what is needed.
Hopefully that can help steer you into the right direction. If you need more help just let us know what you are having trouble with and I am sure we can help give some pointers on good resources or provide some code examples. Thought remember the more information you provide on what you are having trouble with the more specific we can be.
EDIT: Another great resource would be looking at the FTP RFC which will greatly help you with determining what is needed. https://www.ietf.org/rfc/rfc959.txt