You can cite the Real Statistics software in a publication as follows:
“The data analysis for this paper was generated using the Real Statistics Resource Pack software (Release 5.4). Copyright (2013 – 2018) Charles Zaiontz. www.real-statistics.com.”
The above citation uses the latest software release. If you used an earlier release, simply use that number. This number can be found using the =VER() function in the software.
You can reference the Real Statistics website as follows:
Zaiontz C. (2018) Real Statistics Using Excel. www.real-statistics.com
Since the website changes all the time as I add new information, you should also add the date(s) when you accessed the website.