If you are on an earlier major version, such as High Sierra or Mojave, and all you are interested in is how to "block updates" to Catalina, ie prevent the upgrade from automatically occuring, then you don't need to do anything.
At least as of Sep 2020, Apple does not force major version upgrades. Even if you have Sys Prefs > Software Upgrade > Automatically keep my Mac up to date checked, or all of the options under Advanced (including Install MacOS updates) checked, your machine will not will automatically install Catalina.
Apple has gotten very naggy about pushing the Catalina upgrade lately, even deprecating the ability to ignore the update prompt, per @Robughblah's answer. But you still have to click on 'Upgrade Now' in the Catalina upgrade prompt in System Updates for it to occur.
If by "block updates", you are interested not just in preventing the upgrade, but in preventing prompts to upgrade to Catalina from appearing in Software Updates at all, Apple have made that harder, but there is still a way, outlined in my answer here.