HI stacking has proven to be a highly effective tool to statistically analyse average HI properties for samples of galaxies which may or may not be directly detected. With the plethora of HI data expected from the various upcoming HI surveys with the SKA Precursor and Pathfinder telescopes, it will be helpful to standardize the way in which stacking analyses are conducted. In this work we present a new python-based package, HISS, designed to stack HI (emission and absorption) spectra in a consistent and reliable manner. As an example, we use HISS to study the HI content in various galaxy sub-samples from the NIBLES survey of SDSS galaxies which were selected to represent their entire range in total stellar mass without a prior colour selection. This allowed us to compare the galaxy colour to average HI content in both detected and non-detected galaxies. Our sample, with a stellar mass range of 10^8^<M_*_(M_{sun}_)<10^12^, has enabled us to probe the HI-to-stellar mass gas fraction relationship more than half an order of magnitude lower than in previous stacking studies.