You must first create the partition from a live system (such as Trisquel's
live ISO) and format it (ext4 seems to be your filesystem of choice). GParted
(that uses GNU 'parted' and 'mkfs' behind the scene) can do that and is
present on Trisquel's live ISO. Remember the number of the partition.
Then you must add a line into /etc/fstab (you can do that from the live
system or after rebooting in the installed system). The format is detailed in
'man fstab'. It must look something like that:
/dev/sda7 ext4 defaults 0 2
The first column specifies the partition (there is a more robust way to
specify it but the file representing the device is OK in almost all cases),
then the filesystem, then the option passed to the 'mount' command when the
the filesystem will be mounted at init (here the "defaults"), then 0 if the
filesystem needs not be dumped (otherwise 1) and finally the order in which
the filesystem needs to be checked by 'fsck' (0 is it needs not be checked
what you do not want; 1 for the filesystem mounted at /; 2 for the other
filesystems).
