HI,
Is there something in particular in the instructions that are not clear?
A high level overview of the process is to purge the SharePoint history list data and then purge the same data from the Nintex content database. Ie purge all data prior to 2021 for all history lists and in the Nintex content database.
Nintex provided the NWAdmin tool to help with the purging.
Note that the 10 million figure is not a hard figure. It depends on the environment such as how many servers are in the farm and the processing power of those servers. How many users and how many workflows are running. Some farms can get up to 300 million rows and have no issues until they hit 301 million rows. In saying that, SharePoint should not be used for storage and that is why history lists need to be maintained.