In order to do what you want, you would have to first turn on the guest network feature on your "main" AirPort Extreme and get that working correctly first.
Then, when you add the second AiPort Extreme, the setup "wizard" will automatically extend both the private and guest networks using Ethernet with the second AirPort Extreme. You don't have to worry about jargon like "roaming" or "create" or "bridge mode".
The end result is that you will have greater wireless coverage for both the "private" and "guest" networks. They will still be completely separate in that devices on the guest network will not be able to "see" any devices on the "private" network and vice versa.
However, if you do not turn on the guest feature on the "main" AirPort Extreme first.....then you will not be able to set up a "guest" network on the second AirPort Extreme and have it do what you want.
I think that part of the problem that you appear to be having with understanding all of this is that you are looking at a 3+ year old Apple support document that has little to no relation to the way that newer operating systems and newer versions of AirPort Utility operate. Unfortunately, Apple has not updated the document to include the newer operating systems and AirPort Utility versions.
The bottom line here is that if you get the "main" AirPort Extreme working with both the "private" and "guest" networks, then it will take about 2 minutes to set up the second AirPort Extreme to do the same thing.......virtually automatically. All that you need to do is connect the AirPorts using the Ethernet cable, start up the setup wizard, and assign a device name to the second Extreme. The setup wizard takes care of all the rest, you just sit back watch the setup happen on your screen.
If you want step by step, we can provide that.