Note: it is similar to Creating a Ceph OSD from a designated disk partition but simpler.
In a nutshell, to use the remaining space from /dev/sda and assuming Ceph is already configured in /etc/ceph/ceph.conf it is enough to:
$ sgdisk --largest-new=$PARTITION --change-name="$PARTITION:ceph data" \ --partition-guid=$PARTITION:$OSD_UUID \ --typecode=$PARTITION:$PTYPE_UUID -- /dev/sda $ partprobe $ ceph-disk prepare /dev/sda$PARTITION $ ceph-disk activate /dev/sda$PARTITION