The first step should be asking yourself why you feel the need to do it. There are so many mature and FREE packages out there that I would look for something else to do...
I think
FileZilla is still free, and last time I checked, you could even get the source code for it.
EDIT---------------------
FileZilla should be on SourceForge. Google is your friend.
EDIT
Filezilla[
^]
/EDIT