Well to be honest not that easy if you haven’t ever done something like this but check this out
Details regarding how to access the command line interface can be found here on our Wiki: Accessing the command line - General - OSMC
Than use sudo nano /etc/fstab to edit the file. Add the line at the end and leave via Ctrl-x.
Reboot
Be careful you can make OSMC unbootable if you break the fstab file.
Easier might be adding the source to sources.xml as @dandnsmith wrote