If you want to do it manually:
First you have to configure the SSH service on your FreeNAS and create a user for it (consult the manual).
Then something along this line should do:
Code:
rsync -avh /home/<username>/ <ssh-user>@<freenas-ip>:/mnt/<poolname>
Replace the entries for <username>, <ssh-user>, <freenas-ip> and <poolname>
However, I'd recommend you familiarize yourself with the
manual of rsync(1) to understand what the command actually does.
If you don't want to learn the basics of rsync or are uncomfortable using the shell, there exist multiple backup softwares with a GUI supporting many different protocols (SCP, SFTP, FTP, Samba, ...).