Take at least 5 sky flats. The telescope tracking must be on during the flat field exposures. Be aware that as the sky darkens, you will need to increase the exposure time to keep the signal level in the desired range. Remember to turn off the tracking between exposures, and to turn it back on before starting the next exposure. A star that is barely visible in the finder telescope is an appropriate brightness to yield a several second exposure.
Accurate focus is necessary for good stellar images over the entire field.
Take two 5-minute dark exposures. The star cluster is much larger than the CCD field of view, but centering the cluster in the finder telescope should get the central region of the cluster into the CCD field. Try to get several of the brightest stars in the field. Determine the longest exposure time that does not saturate the brightest stars in the field. Take three exposures of this length.