I search "Sips isotherm" and find the following link, http://micro-report.com/isotherm-models/sipps/
We usually plot the isotherms of Langmuir monolayers by using "Microcal Origin" (6.0 or other editions). I think you can also use it to plot the Sips isotherm. The software can be download freely.
Definitely use a software that helps to draw graphs of functions, e.g. Excel or Grapher. But you can also use graph paper with a millimeter or logarithmic scale applied to it. In this way, charts have been drawn by hand over the years.