Please make a measurement or estimate of the coma diameter at the same time and with the same instrument as the magnitude estimate. This is very important for the analysis of the observations as the coma diameter also gives information about your observing conditions.
Always measure the magnitude, coma diameter and DC with the same instrument (which may be the naked eye, binoculars or telescope) and report this instrument. If you make additional measurements of magnitude, coma diameter etc with different apertures, report them separately. I can use these observations to help quantify the aperture effect.
Using the smallest aperture and magnification that show the comet clearly gives more consistent results.
You can measure the tail or coma details with whatever instrument is most suitable. Note the aperture, magnification, seeing etc as with any other astronomical observation.
Some observers claim to be making magnitude estimates of the zero magnitude comet with 20 cm reflectors. I hope they are not!
With a bright comet two problems must be avoided:
1. For a comet with a long tail the CCD should be used with an ordinary camera lens, prefereably a wide angle one to get the maximum coverage of tail detail.
2. For a comet with a bright central condensation very short exposures are needed to avoid saturating the pixels. A good image scale is needed to resolve near nucleus features.
Sadly many images of Hyakutake are over exposed and only show tail detail near the nucleus.
Uwe Schmidtmann has provided some helpful information on how to image features in the cometary coma using rotation or logarithmic scaling.