Each visibility has spectral-line and continuum components. You must remove the continuum to be able to study the spectral-line accurately. In § 11 I described how to make a bandpass calibration; this must be applied before you attempt to subtract the continuum. There are several continuum subtraction methods now available in AIPS. I will discuss them below in quasi-historical order, but I suggest that you read about all the methods before having a go.
I also recommend that you consider the visibility-domain subtraction method as your first line of approach; this is the last method I discuss, and it is probably the least intuitively obvious. However, it is the easiest to use, the fastest, and the most accurate.