As root, in a terminal, use the following command (you can copy and paste into the terminal):
mcedit /etc/fstab Copy this line:
tmpfs /tempdir tmpfs rw,suid,dev,exec,auto,nouser,async,noatime 0 0
In the terminal, scroll down to the section that looks like this:
# Shared memory filesystem:
tmpfs /dev/shm tmpfs rw,suid,dev,exec,auto,nouser,async,noatime 0 0
Paste new line below the one above, using key combination "Shift+Insert", so it looks like this:
# Shared memory filesystem:
tmpfs /dev/shm tmpfs rw,suid,dev,exec,auto,nouser,async,noatime 0 0
tmpfs /tempdir tmpfs rw,suid,dev,exec,auto,nouser,async,noatime 0 0
Press F2 to save, F10 to exit.
Issue command "mount -a"
Done. On subsequent boots it will mount automatically.