* Windows gives you a helpful little wizard wherein you click "next" a few times.
* Firefox gives you a dialog with 3 checkboxes; check them and click okay.
* iOS sends you to settings, and asks you if you want to trust the given CA.
* OS X hands it to Keychain Access, where you have to select 'trust' from a dropdown and maybe enter a keychain password; it's a bit less intuitive.
* Chrome uses the OS trust store, so it hands it off to the OS while claiming it's a dangerous filetype.