2nd you will have to forward the FTP port (TCP 21) on your router to your MBL. This step depends on your router, since all brands are different, refer to your manual.
3rd, you will need to find what your internet/public IP is. Chances are you are on DHCP with your ISP. So it might change every 24-48hrs. You might need to find a way to be notified every time this changes. There are from apps you install, to DDNS, which is the recommeded way. For DDNS it will depend whether your router supports it.
Do baby steps, make sure you undestand #1, before going into #2 and so on.
Good luck, do some internet searches as well for any term you might not understand.