The following instructions are for iOS, but the process for macOS (where Big Sur or higher is required) is very similar. Use Safari when doing this setup because Apple makes profile installation a little harder for other browsers.
Once downloaded you'll see a message about Profile installation.
Go to Settings. Tap the entry labeled Profile Downloaded.
Install the profile. You may see a message about the author being unknown since the file isn't signed, but you can dismiss that.
You're done! (Note: You only need to install the profile once, regardless of any changes made after the fact to settings like category blocks, blacklists/whitelists, etc.)