If you have the cdrom in postition /mnt/cdrom, you need to mount it first.
change to that directory
Now you can make the iso. In this case, just a data iso and not a bootable one....
mkisofs -R -J -o /home/mycd.iso .
Note: In this case, the period ( . ) is important.
If you want to make a bootable cdrom from the iso, you will need a boot image in there.