The first step in creating a Hubble diagram is to plot the distances to several galaxies. Unfortunately, measuring distances in astronomy is extremely difficult; but fortunately, all you need for a Hubble diagram are relative distances to galaxies, not necessarily their actual or "absolute" distances measured in miles or light-years. Relative distances are measured with respect to a convenient but arbitrary standard like the Andromeda galaxy or the Virgo cluster: we can say that the Perseus Cluster is five times the distance of the Virgo Cluster, for example.